Methods for producing and using perovskite materials and devices therefrom

ABSTRACT

The present disclosure describes solution methods for manufacturing perovskite halide films for use in solar cells. The methods include the use of additives that facilitate the formation of transitory, intermediate films that are later transformed into the final target perovskite halide films, such that the final films provide improved physical characteristics and operational performance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/004,020, filed May 28, 2014; U.S. Provisional Application No.62/061,475, filed Oct. 8, 2014; U.S. Provisional Application No.62/103,277, filed Jan. 14, 2015; and is a national stage entry ofInternational Application No. PCT/US15/33068, filed on May 28, 2015; thecontents of each of which are incorporated herein by reference in theirentirety.

CONTRACTUAL ORIGIN

The United States Government has rights in this disclosure underContract No. DE-AC36-08GO28308 between the United States Department ofEnergy and the Alliance for Sustainable Energy, LLC, the Manager andOperator of the National Renewable Energy Laboratory.

BACKGROUND

Perovskite halides (e.g., CH₃NH₃PbI₃) represent a new class of lightabsorbers that have demonstrated tremendous progress in solar cellperformance from 3.8% in 2009 to 20.1% in 2015. One attraction forperovskite halides is their bandgap tuning capability. For example,perovskite halide CH₃NH₃PbI₂Br has a bandgap of about 1.8 eV, whichmakes it suitable for developing tandem or multijunctionperovskite-based solar cells. However, perovskite halides have thecapability of bandgap tuning by modifying their components buildingblocks. Moreover, high-efficiency perovskite solar cells can be made bylow-cost processing methods. In addition, perovskite halides are notonly direct-bandgap absorbers with strong absorption coefficients, butthey also demonstrate long electron and hole diffusion lengths. Forthese and other reasons, perovskite halide solar cell technology isexpected to attract strong interest from solar manufacturers andstart-ups looking for their next-generation solar cell products.

However, current manufacturing methods typically produce unsatisfactoryperovskite halide films. For example, many of the films produced are notfull coverage, continuous films, free from pin-holes. In addition, it isvery difficult to produce perovskite halide films that are highlycrystalline and phase-pure; e.g. films that do not possess residualimpurities such as lead iodide. Thus, there remains a need for bothbetter performing perovskite halide films, and solar cells madetherefrom, but also improved manufacturing methods for making highperformance perovskite halide films.

SUMMARY

An aspect of the present invention is a “two-step” method of forming aperovskite film, where the method includes applying a solution onto asubstrate, the solution containing at least two alkyl ammonium halidesand a metal halide in a solvent. The method then includes thermallytreating the solution, such that the treating removes substantially allof the solvent and at least a portion of at least one of the alkylammonium halides from the solution, and the remaining alkyl ammoniumhalides and the metal halide crystallize to form a final alkyl ammoniummetal halide perovskite film on the substrate.

In some embodiments of the present invention, the solution may includetwo alkyl ammonium halides. The first alkyl ammonium halide may be analkyl ammonium chloride and the second alkyl ammonium halide may be analkyl ammonium iodide.

In some embodiments of the present invention, during the treating, anintermediate alkyl ammonium metal halide perovskite film may form wherethe intermediate film may include at least two halogen elements, whichthen transform to the final perovskite film that includes at least onehalogen element. In other embodiments, the intermediate perovskite filmmay include two halogen elements and the final perovskite film comprisesone halogen element. In still further embodiments, the two halogenelements may be chlorine and iodine and the one halogen element may beiodine.

In some embodiments of the present invention, the intermediateperovskite film may include at least one of a methyl ammonium metaliodide and a methyl ammonium metal chloride, or a methyl ammonium metaldihalide comprising iodide and chloride. In some embodiments of thepresent invention, the final perovskite film may be a methyl ammoniummetal iodide.

In some embodiments of the present invention, the alkyl group for the atleast two alkyl ammonium halides may be at least one of a methyl group,an ethyl group, a propyl group, or a butyl group. In some embodiments ofthe present invention, each of the at least two alkyl ammonium halidesmay have a halogen from at least one of fluorine, chlorine, bromine,iodine, or astatine, such that the halogen for each alkyl ammoniumhalide is different from the halogen of each of the remaining alkylammonium halides. In some embodiments, a first alkyl ammonium halide maybe alkyl ammonium chloride with a second alkyl ammonium halide with ahalogen selected from bromine, iodine, or astatine.

In some embodiments of the present invention, a metal of the metalhalide may be at least one of lead, tin, germanium, or any other metalin the 2+ valence state. In some embodiments of the present invention,the solvent may be at least one polar solvent. In still furtherembodiments of the present invention, the solvent may be at least one ofdimethysulfoxide, dimethylformamide, γ-butyrolactone, orN-methyl-2-pyrrolidone.

In some embodiments of the present invention, the metal halide and thealkyl ammonium iodide may be present in the solution at a molar ratio ofabout one to one (1:1). In still further embodiments, the alkyl ammoniumiodide and the alkyl ammonium chloride may be present in the solution ata molar ratio ranging from about 1:0.1 to about 1:4. In still furtherembodiments, at least one of the alkyl ammonium halides, or the metalhalide may be present in the solution at a concentration ranging fromabout 0.1 M to about 3.0 M.

In some embodiments of the present invention, the thermally treating mayinclude heating the solution at a temperature ranging from about 20° C.to about 250° C. In some further embodiments of the present invention,the thermally treating comprises heating the solution for a period oftime ranging from about 30 seconds to about 6 hours.

A further aspect of the present invention is a “three-step” method offorming a perovskite film, where the method includes applying a firstsolution onto a substrate, the first solution containing in a firstsolvent, a first alkyl ammonium halide with a first halogen, and a metalhalide with a second halogen different from the first halogen. Themethod then includes completing a first thermal treatment of the firstsolution to form a first film of the metal halide on the substrate,followed by contacting a second solution with the first film, the secondsolution containing in a second solvent, a second alkyl ammonium halidewith the second halogen, such that the second alkyl ammonium halidereacts with the first film to form a final crystalline alkyl ammoniummetal halide perovskite film that is substantially free of the firstfilm and the first halogen.

In some embodiments of the present invention, the “three-step” methodmay further include, during at least a portion of the contacting,completing a second thermal treatment of the second solution and thefirst film. In some embodiments of the present invention, the firstthermal treatment may form an unstable intermediate film that mayinclude the metal halide and an alkyl ammonium metal halide with thefirst halogen, such that the intermediate film subsequently changes toform the first metal halide film.

In some embodiments of the present invention, the first halogen and thesecond halogen may include fluorine, chlorine, bromine, iodine, orastatine. In other embodiments, the first halogen may have a lowermolecular weight than the second halogen. In still further embodiments,the first halogen is chlorine and the second halogen is iodine.

In some embodiments of the present invention, an alkyl group for atleast one of the first alkyl ammonium halide or the second alkylammonium halide may include at least one of a methyl group, an ethylgroup, a propyl group, or a butyl group. In some embodiments of thepresent invention, a metal of the metal halide may include at least oneof lead, tin, germanium, or any other metal in the 2+ valence state.

In some embodiments of the present invention, the first solvent mayinclude at least one polar solvent. In some further embodiments, thesecond solvent comprises isopropyl alcohol.

In some embodiments of the present invention, the metal halide and thefirst alkyl ammonium halide may be present in the first solution at amolar ratio ranging from about 1:0.1 to about 1:3. In some furtherembodiments, at least one of the metal halide and the first alkylammonium halide may be present in the first solution at a concentrationranging from about 0.1 M to about 3.0 M. In still further embodiments,the second alkyl ammonium halide may be present in the second solutionat a concentration ranging from 0.1 mg/ml solvent to about 100 mg/mlsolvent.

In some embodiments of the present invention, at least one of thethermal treatments may include heating at least one of the solutions ata temperature ranging from about 40° C. to about 250° C. In furtherembodiments, at least one of the thermal treatments may include heatingat least one of the solutions for a period of time ranging from about 30seconds to about 6 hours.

A further aspect of the present invention is a method for manufacturinga solar cell, the method including in order, forming a perovskite filmon a substrate by any one of the methods described herein, depositing ahole-transport layer on the perovskite film, depositing a molybdenumoxide layer on the hole-transport layer, and depositing an aluminumlayer on the molybdenum oxide layer.

A further aspect of the present invention is a solar cell including inorder, transparent conducting oxide substrate, an electron-transportlayer on the transparent conducting oxide substrate, a perovskite halidefilm on the electron-collecting layer, a hole-transport layer on theperovskite halide film, a molybdenum oxide layer on the hole-transportlayer, and an aluminum layer on the molybdenum oxide layer.

In some embodiments of the present invention, the electron-transportlayer may be titanium oxide. In further embodiments of the presentinvention, the hole-transport layer may be spiro-OMeTAD.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an alkyl ammonium metal halide perovskite crystalstructure, according to exemplary embodiments of the present invention.

FIG. 2 illustrates a solar cell constructed with a perovskite halidefilm and molybdenum oxide/aluminum top contact, according to exemplaryembodiments of the present invention.

FIG. 3 illustrates a schematic of a “one-step” method for making amethyl ammonium lead iodide perovskite, according to exemplaryembodiments of the present invention.

FIG. 4 illustrates a schematic of a “one-step” method for making amethyl ammonium lead iodide bromide perovskite, according to exemplaryembodiments of the present invention.

FIG. 5 illustrates a schematic of a “three-step” method for making analkyl ammonium metal halide perovskite, according to exemplaryembodiments of the present invention.

FIG. 6 provides images of the perovskite films prepared from CH₃NH₃PbI₃precursors with different amounts of CH₃NH₃Cl and annealed at 100° C.with varying duration.

FIGS. 7a-d are UV-vis absorption spectra of perovskite CH₃NH₃PbI₃ onmesoporous TiO₂ films as a function of annealing time at 100° C. usingprecursors containing (a) 0 methyl ammonium chloride (“MACl”), (b) 0.5MACl, (c) 1 MACl, and (d) 2 MACl.

FIGS. 8a-c depict (a) XRD patterns of CH₃NH₃PbI₃ on mesoporous TiO₂ filmas a function of the MACl amount in the precursors for differentannealing times at 100° C.; magnified views of the XRD patterns (b) near14 degrees for perovskite films using 0-2 MACl with their respectiveoptimum annealing time; and magnified views of the XRD patterns (c) forthe 2-MACl sample with different annealing time (1-45 min).

FIGS. 9a-h are typical SEM images of perovskite CH₃NH₃PbI₃ grown on(a-d) mesoporous TiO₂ film and (e-h) planar TiO₂ compact layer withdifferent MACl amount (a, e: 0; b, f: 0.5; c, g: 1; d, h: 2).

FIGS. 10a-b are graphs showing the effect of MACl on the J-V curves of(a) mesostructured and (b) planar perovskite CH₃NH₃PbI₃ solar cells.

FIGS. 11a-b are graphs showing the effect of MACl on the recombinationresistance (R_(rec)) as a function of voltage for (a) mesostructured and(b) planar perovskite CH₃NH₃PbI₃ solar cells.

FIGS. 12a-b are dark J-V curves of (a) mesostructured and (b) planarperovskite solar cells as a function of added amount of MACl in theprecursor solution for growing CH₃NH₃PbI₃.

FIG. 13 is a graph showing the effect of annealing time (at 100° C.) onthe XRD patterns of perovskite films prepared from precursor containingequimolar PbI₂ and MACl. In addition to the main perovskite (110) peaknear 14 degrees, two new peaks appear at about 12.6 (denoted by pound)and 15.5 degrees (denoted by asterisk).

FIGS. 14a-b are graphs displaying the effect of MACl on (a) electrondiffusion coefficient as a function of photoelectron density and (b)recombination lifetime as function of voltage in mesostructuredperovskite CH₃NH₃PbI₃ solar cells.

FIG. 15 is a graph displaying UV-vis absorption spectra of perovskitefilm as a function of annealing time at 100° C. using a precursorcontaining equimolar mixture of MACl and PbI₂.

FIGS. 16a-c are (a) SEM image, (b) XRD patterns, and (c) EDX analysis ofthe mixed halide CH₃NH₃PbI_(3-x)Cl_(x) prepared from the precursorcontaining MAI and PbCl₂ (3:1 molar ratio).

FIG. 17 is a graph showing recombination resistance R_(rec) as afunction of voltage for a planar perovskite solar cell based on themixed halide CH₃NH₃PbI_(3-x)Cl_(x) prepared from the precursorcontaining MAI and PbCl₂ (3:1 molar ratio).

FIG. 18 is a J-V curve of a planar perovskite solar cell based on themixed halide CH₃NH₃PbI_(3-x)Cl_(x) prepared from the precursorcontaining MAI and PbCl₂ (3:1 molar ratio). The cell efficiency is11.86% with a J_(sc) of 20.57 mA/cm², V_(oc) of 0.995 V, and FF of0.579.

FIGS. 19a-c are typical SEM images of MAPbI₂Br films deposited onc-TiO₂/FTO substrates from various precursor solutions; (a) PbI₂+MABr;(b) PbI₂+MABr+MACl; and (c) PbI₂+MABr+2MACl.

FIGS. 20a-c are graphs showing the effect of annealing time (asindicated) at 125° C. on the XRD patterns of MAPbI₂Br films deposited onc-TiO₂/FTO substrate from various precursor solutions: (a) PbI₂+MABr;(b) PbI₂+MABr+MACl; and (c) PbI₂+MABr+2MACl.

FIG. 21 is a graph showing the photocurrent density-photovoltage (J-V)curves of perovskite solar cells prepared from different precursorcompositions (i.e., PbI₂+MABr or 0 MACI; PbI₂+MABr+MACl or 1 MACl; andPbI₂+MABr+2MACl or 2MACl).

FIGS. 22a-b are graphs showing the effect of the dipping time in the 10mg MAI/mL IPA solution on the evolution of (a) XRD patterns and (b)UV-vis absorption spectra for the perovskite films deposited from 1.0 MPbI2 precursor on mesoporous TiO₂ film.

FIGS. 23a-b are graphs showing the effect of PbI₂ concentration (0.4-1.0M) on the (a) XRD patterns and (b) UV-vis absorption spectra of theperovskite films deposited on mesoporous TiO₂ film after dipping in 10mg MAI/mL IPA solution for 10 min.

FIGS. 24a-b are graphs showing (a) UV-vis absorption spectra and (b) XRDpatterns of the films in each of the three production steps: (1) initialfilm formation from precursor of mixed PbI₂ and MACl; (2) thermallydecomposed PbI₂ film from the second step; (3) final CH₃NH₃PbI₃ formedafter dipping in MAI solution after the third step.

FIG. 25 is a curve showing the J-V characteristic of a perovskite solarcell based on the CH₃NH₃PbI₃ film prepared from the three-step approach.

FIGS. 26a-d are (a) UV-vis absorption spectra and (b) XRD patterns ofCH₃NH₃PbI₃ films grown via the standard two-step sequential solutiondeposition using pure PbI₂ precursor film with different MAI solutiondipping times. SEM images of (c) the typical spin-coated PbI₂ film and(d) the converted CH₃NH₃PbI₃ film with 20-min dipping (reaction) time inMAI solution are also depicted.

FIG. 27 depicts typical J-V curves of planar CH₃NH₃PbI₃ solar cellsprepared from PbI₂ precursor films with different MAI solution dippingtime from 0.5 to 20 min.

FIGS. 28a-d are (a) UV-vis absorption spectra and (b) XRD patterns ofPbI₂.xMAI films (with varying x values as indicted) before the secondconversion step; (c) UV-vis absorption spectra of CH₃NH₃PbI₃ filmsprepared from PbI₂.xMAI films after the second conversion step; (d)evolution of UV-vis absorption spectra using the PbI₂.0.15MAI precursorfilm with different dipping times in the MAI solution.

FIG. 29 is a graph showing XRD patterns of PbI₂.xMAI (x: 0.1-0.3) filmsafter dipping in MAI solution for 1-3 min.

FIGS. 30a-h are typical SEM images of PbI₂.xMAI (x=0.1, 0.15, 0.2, and0.3) precursor films (a-d) and CH₃NH₃PbI₃ films prepared from theirrespective precursor films as indicated (e-h).

FIGS. 31a-b are (a) a schematic illustration of the crystal phasetransformation from PbI₂ to CH₃NH₃PbI₃ in the standard two-stepsequential deposition process and (b) a plot of the film thickness (orvolume) expansion ratios of converting PbI₂.xMAI to CH₃NH₃PbI₃ films asa function of the relative amount x MAI used. The solid line is the bestfit as discussed in the Detailed Description.

FIG. 32 is a graph showing the typical J-V curves of planar CH₃NH₃PbI₃solar cells prepared from PbI₂.xMAI precursor films.

FIG. 33 shows plots of recombination resistance R_(rec) as a function ofvoltage for solar cells prepared with different PbI₂.xMAI precursorcompositions (x: 0-0.3).

FIGS. 34a-b are (a) the J-V curve of a planar CH₃NH₃PbI₃ solar cellprepared from PbI₂.0.15MAI precursor with the highest efficiency of16.76% (J_(sc)=20.86 mA/cm², V_(oc)=1.09 V, FF=0.737) under simulatedone-sun illumination; and (b) the stability of power conversionefficiency as a function of time for the same cell in (a) undersimulated one-sun illumination.

FIGS. 35a-c are the sequence (a) to (c) of the color change of a(CH₃NH₃)PbI₃ film (deposited on a mesoporous TiO₂ film on FTO) passingthrough the top opening of a 3% NH₃ solution bottle.

FIG. 36a-b are (a) UV-vis absorption spectra of a (CH₃NH₃)PbI₃ film inthe absence and in the presence of NH₃ gas; and (b) the photoresponse(transmitted light intensity) of a (CH₃NH₃)PbI₃ film at 660 nm asmeasured by a silicon detector in response to the introduction (on) andremoval (off) of NH₃ gas.

FIG. 37 depicts XRD patterns of the TiO₂/FTO substrate (black line), aPbI₂ film on TiO₂/FTO, a (CH₃NH₃)PbI₃ film on TiO₂/FTO, and theNH₃-treated (CH₃NH₃)PbI₃ film on TiO₂/FTO. The asterisks denote thecharacteristic XRD peaks for (CH₃NH₃)PbI₃.

FIG. 38 depicts XRD patterns and UV-vis absorption spectra of perovskiteCH₃NH₃PbI₃ deposited on a compact TiO₂ layer on the FTO substrate.

FIGS. 39a-b illustrate (a) typical cross-sectional SEM image of a planarFTO/TiO₂/CH₃NH₃PbI₃/spiro-MeOTAD/MoO_(x)/Metal (Ag or Al) solar cell;and (b) the effect of MoO_(x) interlayer thickness on the J-Vcharacteristics of perovskite CH₃NH₃PbI₃ solar cells using Ag topcontact.

FIGS. 40a-b are a comparison of (a) J-V characteristics and (b) IPCE ofperovskite CH₃NH₃PbI₃ solar cells using different top-contact structuresas indicated.

FIGS. 41a-b are (a) typical Nyquist plots of the impedance responses (Z)for a planar perovskite cell (using HTM/10-nm MoO_(x)/Ag) with threedifferent bias voltages; and (b) a comparison of recombinationresistance (R_(rec)) as a function of voltage for planar perovskitesolar cells using different top-contact structures.

FIG. 42 shows the pictures of the perovskite solar cells after 15 hourcontinuous light soaking under simulated one sun illumination.

FIG. 43 shows the dependence of solar cell efficiencies as a function ofcontinuous light soaking under simulated one sun illumination for 15hours.

DETAILED DESCRIPTION

FIG. 1 illustrates that perovskite halides 100 may organize into cubiccrystalline structures and may be described by the general formula ABX₃,where X (130) is an anion and A (110) and B (120) are cations, typicallyof different sizes (A typically larger than B). In a cubic unit cell,the B-cation 120 resides at the eight corners of a cube, while theA-cation 110 is located at the center of the cube with twelve X-anions130 centrally located between B-cations 120 along each edge of the unitcell. Typical inorganic perovskites include calcium titanium oxide(calcium titanate) minerals such as, for example, CaTiO₃ and SrTiO₃. Insome embodiments of the present invention, the A-cation 110 may includea nitrogen-containing organic compound such as an alkyl ammoniumcompound. The B-cation 120 may include a metal and the X-anion 130 mayinclude a halogen.

Additional examples for A-cations 110 include organic cations and/orinorganic cations. Organic A-cations 110 may be an alkyl ammoniumcation, for example a C₁₋₂₀ alkyl ammonium cation, a C₁₋₆ alkyl ammoniumcation, a C₂₋₆ alkyl ammonium cation, a C₁₋₅ alkyl ammonium cation, aC₁₋₄ alkyl ammonium cation, a C₁₋₃ alkyl ammonium cation, a C₁₋₂ alkylammonium cation, and/or a C₁ alkyl ammonium cation. Further examples oforganic A-cations 110 include methylammonium (CH₃NH³⁺), ethylammonium(CH₃CH₂NH³⁺), propylammonium (CH₃CH₂CH₂NH³⁺), butylammonium(CH₃CH₂CH₂CH₂NH³⁺), formamidinium (NH₂CH═NH²⁺), and/or any othersuitable nitrogen-containing organic compound. In other examples,A-cations 110 may include an alkylamine. Thus, A-cations 110 may includean organic component with one or more amine groups. For example, anA-cation 110 may be an alkyl diamine halide such as formamidinium(CH(NH₂)₂).

Examples of metal B-cations 120 include, for example, lead, tin,germanium, and or any other 2+ valence state metal that cancharge-balance the perovskite halide 100. Examples for the X-anion 130include halogens: e.g fluorine, chlorine, bromine, iodine and/orastatine. In some cases, the perovskite halide may include more than oneX-anions 130, for example pairs of halogens; chlorine and iodine,bromine and iodine, and/or any other suitable pairing of halogens. Inother cases, the perovskite halide 100 may include two or more halogensof fluorine, chlorine, bromine, iodine, and/or astatine.

Thus, A-cation 110, B-cation 120, and X-anion 130 may be selected withinthe general formula of ABX₃ to produce a wide variety of perovskitehalides 100, including, for example, methylammonium lead triiodide(CH₃NH₃PbI₃), and mixed halide perovskites such as CH₃NH₃PbI_(3-x)Cl_(x)and CH₃NH₃PbI_(3-x)Br_(x). Thus, a perovskite halide 100 may have morethan one halogen element, where the various halogen elements are presentin none integer quantities; e.g. x is not equal to 1, 2, or 3. Inaddition, perovskite halides, like other organic-inorganic perovskites,can form three-dimensional (3-D), two-dimensional (2-D), one-dimensional(1-D) or zero-dimensional (0-D) networks, possessing the same unitstructure.

As stated above, the A-cation 110 may include an organic constituent incombination with a nitrogen constituent. In some cases, the organicconstituent may be an alkyl group such as straight-chain or branchedsaturated hydrocarbon group having from 1 to 20 carbon atoms. In someembodiments, an alkyl group may have from 1 to 6 carbon atoms. Examplesof alkyl groups include methyl (C₁), ethyl (C₂), n-propyl (C₃),isopropyl (C₃), n-butyl (C₄), tert-butyl (C₄), sec-butyl (C₄), iso-butyl(C₄), n-pentyl (C₅), 3-pentanyl (C₅), amyl (C₅), neopentyl (C₅),3-methyl-2-butanyl (C₅), tertiary amyl (C₅), and n-hexyl (C₆).Additional examples of alkyl groups include n-heptyl (C₇), n-octyl (C₈)and the like.

FIG. 2 illustrates a solar cell 200 constructed with a perovskite halideabsorbing layer 240. In this example, the perovskite halide absorbinglayer 240 is positioned between an electron-transport layer 250 and ahole-transport layer 230. In some cases, the electron-transport layer250 may be constructed from titanium dioxide and the hole-transportlayer 230 may be constructed from Spiro-OMeTAD(2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene).In addition, FIG. 2 illustrates that the solar cell 200 has a bottomcontact layer 260 positioned on a transparent substrate 270. The bottomcontact layer 260 may be any suitable transparent conducting oxidelayer, for example fluorine-doped tin oxide. The transparent substrate270 may be glass or any other suitable material. FIG. 2 also illustratesthat the solar cell 200 may include a top contact constructed from alayer of molybdenum oxide 220 and aluminum 210. Embodiments of thepresent invention similar to that shown in FIG. 2 have demonstratedsuperior results, especially regarding performance stability over time.

In some embodiments of the device summarized in FIG. 2, a solar cellutilizing a molybdenum oxide/aluminum (i.e., MoO_(x)/Al) top contactwith a CH₃NH₃PbI₃ absorber layer demonstrated an efficiency of about11.4%. The molybdenum oxide/aluminum may replace noble/precious metals(e.g., Au or Ag) as a top contact for extracting photogenerated holes.As described in detail herein, the device performance of perovskitesolar cells using a MoO_(x)/Al top contact may be comparable to that ofcells using silver as the standard top contact. In addition, analysis ofimpedance spectroscopy measurements suggests that using a thin (e.g.,10-nm thick) MoO_(x)/Al top contact does not appear to affectcharge-recombination properties of perovskite solar cells.

FIG. 3 illustrates a method 300 of making perovskite halide films. Themethod may begin at 310 with preparing a solution that may include asolvent, a metal halide, and at least two alkyl ammonium halides. Forexample, the solvent may be dimethylformamide (DMF), the metal halidemay be lead iodide (PbI₂) and the two alkyl ammonium halides may bemethyl ammonium iodide (MAI—CH₃NH₃I) and methyl ammonium chloride(MACl—CH₃NH₃Cl).

Once the solution is prepared, it may be deposited or applied 320 to asuitable substrate; e.g. a titanium oxide surface on a transparentconducting oxide layer. The depositing 320 may be accomplished by anysuitable means, for example, spin coating, dip coating, curtain coating,etc. In the process of depositing 320, at least a portion of the solventmay be removed from the solution. The depositing 320 may be followed bythermal treatment 330. Thermal treatment 330 may include heating thesolution to at least one specified target temperature, resulting in theremoval of any remaining solvent and at least one of the alkyl ammoniumhalides. In some cases the thermal treatment 330 may include atemperature ramp, such that the solution is heated over a specifiedrange of temperature over a specified period of time. Alternatively, thesolution may be allowed “dry” in ambient conditions, for example, atroom temperature. For example, referring again to FIG. 3, a solution ofDMF, lead iodide, MAI, and MACl may be heated to about 100° C. where thesolution is maintained at that temperature for about 30 minutes,resulting in the removal of substantially all of the DMF andsubstantially all of the MACl, resulting in the formation 340 of thetarget perovskite halide film on the substrate; methyl ammonium leadhalide perovskite (CH₃NH₃PbI₃). In some cases, the thermal treatmentstep 330 may remove substantially all of the solvent. However, in othercases small amounts of solvent may remain in the final target perovskitefilm.

However, the methods described herein need not be limited to producinghalide perovskites containing only one halogen element. FIG. 4illustrates another example of the present invention, a method 400 thatmay produce a perovskite dihalide (e.g. containing two halogenelements). In this example, the method may begin with preparing 410 asolution containing a metal halide and two alkyl ammonium halides in asolvent (e.g. DMF), where each of these three compounds contain adifferent halogen. For example, the metal halide may be lead iodide, thefirst alkyl ammonium halide may be methyl ammonium bromide (MABr), andthe second alkyl ammonium halide may be MACl. Once the solutionpreparation 410 is completed, it may be applied 420 to a suitablesubstrate (e.g. spin coating). After applying 420, the solution may thenbe thermally treated 430 to form 440 the final perovskite halide film onthe substrate. Referring again to FIG. 4, the final perovskite halidefilm may be methyl ammonium lead di-iodide bromide (CH₃NH₃PbI₂Br).

As will be demonstrated later in this disclosure, the methods summarizedin FIGS. 3 and 4 may provide significantly improved final perovskitehalide films, and better performing solar cells that incorporate thesefilms. Referring again to the example illustrated in FIG. 3, referred toherein as a “one-step” method, the solution may begin with an equimolarmixture of MAI and PbI₂ in the solvent. The use of the second alkylammonium halide, the MACl, strongly affects the crystallization processduring the thermal treatment step 330 leading to the formation 340 ofthe final CH₃NH₃PbI₃ film. The final CH₃NH₃PbI₃ film made by this methoddemonstrates enhanced absorption of light and significantly improvedcoverage of the CH₃NH₃PbI₃ film on its substrate. This method has beenshown to provide measurable improvement relative to some of the standardone-step solution methods used to manufacture CH₃NH₃PbI₃ films. Forexample, usage of MACl as the second alkyl ammonium halide has shownsolar cells performance improvement from about 2% to 12% for a planarcell structure and from about 8% to 10% for a mesostructured devicearchitecture, relative to one-step methods utilizing only one alkylammonium halide (MAI).

FIG. 5 illustrates another exemplary method 500 for forming finalperovskite halide films, according to some embodiments of the presentinvention. This method 500 may begin with preparing 510 a first solutioncontaining a metal halide and an alkyl ammonium halide in a firstsolvent, where the metal halide and the alkyl ammonium halide containdifferent halogens. Once the first solution is prepared, it may beapplied 520 to a substrate. The applying step 520 may be by spincoating, dip coating, curtain coating, and/or any other suitablesolution technique. For the case of spin coating, the applying step 520may remove some or substantially all of the first solvent. Once applied,the solution may be thermally treated 530. The thermal treatment step530 may include heating the solution to a final target temperature andthen maintaining the solution at that target temperature for a definedperiod of time. In some cases the thermal treatment 530 may include atemperature ramp, such that the solution is heated over a specifiedrange of temperature over a specified period of time. Alternatively, thesolution may be allowed to “dry” in ambient conditions, for example, atroom temperature. The thermal treatment step 530 will cause theformation of an intermediate film that includes the metal halide and thealkyl ammonium halide. However, this intermediate film is only atransitory film. Eventually the thermal treating 530 removessubstantially all of the alkyl ammonium halide and transforms 540 theintermediate film into a porous metal halide film. In some cases theremay be significant residual alkyl ammonium halide (e.g., MAI, MACl, orMABr) in the porous (or compact) metal halide film. Next, the metalhalide film may be contacted 550 with a second solution. This secondsolution may include a second alkyl ammonium halide, different from thefirst, in a second solvent, which may be different from the firstsolvent. In some cases, the contacting step 550 will transform 560 themetal halide film into the final target perovskite halide film. In othercases, the contacting step 550 may also include thermally treating thesecond solution, by heating the solution above ambient and/or roomtemperature.

Referring again to FIG. 5, the specific method 500 example shown, beginswith preparing 510 a first solution of lead iodide and MACl in DMFsolvent. The first solution is then applied 520 to a substrate byspin-coating the solution onto a substrate (e.g. titanium dioxide on atransparent conducting oxide film). After the first solution has beenapplied 520 to the substrate, the solution is then thermally treated 530by heating the solution to about 130° C. and maintaining thattemperature for about 30 minutes. The thermal treatment step 530 resultsin the temporary formation of an intermediate film of PbI₂*MACl, whichthen degrades/decomposes/transforms 540 to a porous lead iodide filmthat is substantially free of solvent and MACl. The lead iodide film isthen brought into contact 550 with a second solution prepared from a MAIin a second solvent, in this case isopropyl alcohol (IPA). In this case,the contacting step 550 is for 30 seconds at room temperature. However,in other embodiments, the contacting step 550 may be for longer orshorter periods of time at temperatures above room temperature (e.g. upto 250° C.).

In general the second solvent should have a lower solubility for thefinal target perovskite film than the solubility of the first solventfor the final target perovskite film. IPA is one example, however, othercommon alcohol-containing solvents may also be used; e.g. butyl alcohol.

In some cases the thermal treatment step 530 may remove substantiallyall of the first solvent. However, in other cases small amounts of thefirst solvent may remain in the porous metal halide film. Similarly, insome cases the contacting step 550 may remove substantially all of thesecond solvent. However, in other cases small amounts of the secondsolvent may remain in the final target perovskite film.

The method illustrated in FIG. 5 is referred to herein as a “three-step”method. In summary, the three-step method may be characterized by theformation of a transitory and thermally unstable stoichiometricprecursor film of an PbI₂.alkyl ammonium halide film (e.g. depositedonto a mesoporous TiO₂ substrate), which then during thermal treatment530 transforms to a porous PbI₂ film. In some cases (e.g., “drying” atroom temperature) there may be significant residual alkyl ammoniumhalide in the PbI₂ film. Then, during a second contacting step 550, thePbI₂ film is converted into CH₃NH₃PbI₃ by contacting the PbI₂ film witha solvent solution (e.g. IPA) of CH₃NH₃I. Thus, embodiments of thethree-step method shown in FIG. 5, enable the formation of metal halidefilm, through the thermal decomposition of the metal halide.alkylammonium halide precursor film. This transition from a transitoryintermediate film to a lead halide film enables the rapid conversion ofmetal halide film to the final target perovskite halide film, withoutany residual metal halide in the final target perovskite film, whichresults in improved device performance (e.g. solar cell).

EXAMPLES

The following examples are included to demonstrate certain embodimentsof the present disclosure. It should be appreciated by those of skill inthe art that the techniques disclosed in the examples which followrepresent techniques discovered by the inventors to function well in thepractice of the subject matter of the present disclosure, and thus canbe considered to constitute modes for its practice. However, those ofskill in the art should, in light of the present disclosure, appreciatethat many changes can be made to the specific embodiments disclosedherein and still obtain a like or similar result without departing fromthe spirit and scope of the subject matter of the present disclosure.

Example 1: One Step Method

Disclosed herein is a one-step solution method for preparing perovskiteCH₃NH₃PbI₃ films. The disclosed one-step solution method includesintroducing a second alkyl ammonium halide, MACl, to an equimolarmixture of a first alkyl ammonium halide, MAI, and a metal halide, leadiodide, in solution—to prepare perovskite CH₃NH₃PbI₃ on a substrate.Examples of substrates that may be used include mesoporous and planarTiO₂ substrates. Optical and structural characterizations show that theuse of one or more alkyl ammonium halides adjusts the crystallizationprocess for forming CH₃NH₃PbI₃. Depending on the amount of alkylammonium halides in the precursor solution, the crystallization processmay occur in a time period ranging from a few minutes to several tens ofminutes. Using two or more alkyl ammonium halides not only improvesabsorption of CH₃NH₃PbI₃ but also enhances the CH₃NH₃PbI₃ coverage onplanar substrates, leading to significant improvement of deviceperformance. In this Example, methyl ammonium chloride (MACl) was usedas a second alkyl ammonium halide and was mixed into a solvent solutioncontaining MAI and PbI₂), to generate final target perovskite halidefilms of CH₃NH₃PbI₃. Charge transport and recombination properties wereexamined by intensity-modulated photocurrent/photovoltage spectroscopies(IMPS/IMVS) and impedance spectroscopy (IS).

Experimental Setup and Device Fabrication

Transparent conducting substrate and mesoporous TiO₂ films werefabricated. Initially, fluorine-doped transparent conducting SnO₂-coatedglass substrate was pre-patterned by etching with Zn powder and ˜25 wt %HCl solution for about 2 minutes. The patterned FTO substrate was thencleaned by soaking in ˜5 wt % NaOH in alcohol for ˜16 hours and thenrinsing it sequentially with deionized (DI) water and ethanol. Thecleaned FTO substrate was subsequently coated with a compact TiO₂ layerby spray pyrolysis using 0.2 M Ti(IV) bis(ethylacetoacetate)-diisopropoxide in 1-butanol solution at ˜450° C., followedby annealing at ˜450° C. for about one hour. The 20-nm-sized TiO₂nanoparticles were synthesized utilizing standard methods known in thefield. The TiO₂ paste, together with ˜6 wt % of the 20-nm TiO₂nanoparticles were mixed with terpineol and ethyl cellulose, and thenscreen-printed with 3-μm emulsion thickness on the patterned FTOsubstrates. The printed mesoporous TiO₂ film was annealed at ˜500° C.for about 0.5 hours. The average film thickness was determined by anAlpha-Step 500 surface profiler. The TiO₂ films were then soaked in˜0.04 M TiCl₄ solution at ˜65° C. for about 30 minutes, followed byrinsing with DI water and ethanol, and finally dried under N₂. TheTiCl₄-treated TiO₂ films (for mesostructured cells) and compact TiO₂films (for planar devices) on patterned FTO were annealed again at ˜500°C. for ˜30 minutes before the CH₃NH₃PbI₃ perovskite films were depositedon these substrates.

In connection with device fabrication steps, CH₃NH₃I (MAI) wassynthesized by reacting methylamine (33 wt % ethanol solution) andhydroiodic acid (57 wt % in water, Aldrich) with the molar ratio of1.2:1 in an ice bath for 2 h with stirring followed by vacuum drying andcleaning with ethyl acetate. CH₃NH₃Cl (MAC′) was synthesized by reactingmethylamine (33 wt % ethanol solution) and 33 wt % hydrocholoride acidwith the molar ratio of 1.2:1 in an ice bath for 2 h with stirringfollowed by vacuum drying and cleaning with acetonitrile. Four differentCH₃NH₃PbI₃ precursor solutions were prepared by dissolving 0.693 g PbI₂(1.5 mmol); 0.239 g MAI (1.5 mmol); and 0 g, 0.050 g (0.75 mmol), 0.100g (1.5 mmol), or 0.200 g (3 mmol) MACl in 2.75 g dimethylformamide (DMF)at room temperature—noted as 0 molar (M) MACl, 0.5 M MACl, 1 M MACl, and2 M MACl, respectively. Devices were fabricated in ambient condition(unless stated otherwise) as detailed below. The perovskite CH₃NH₃PbI₃precursor solutions were spin-coated onto (1) 650-nm-thick TiO₂mesoporous films on FTO at 3,000 rpm for ˜30 seconds for mesostructuredcells, and (2) compact TiO₂ films on FTO at 2,500 rpm for ˜10 seconds.The perovskite-coated films were then annealed on a hotplate at ˜100° C.for periods of time ranging from about 5 minutes to about 45 minutes.For both mesostructured and planar perovskite solar cells, ahole-transport material (HTM) solution was spin-coated on theperovskite-covered TiO₂ electrodes at 4,000 rpm for ˜30 seconds. The HTMsolution consisted of 0.1 M2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene(spiro-MeOTAD), 0.035 M bis(trifluoromethane)sulfonimide lithium salt(Li-TFSi), and 0.12 M 4-tert-butylpyridine (tBP) inchlorobenzene/acetonitrile (10:1, v/v) solution. Finally, a 150-nm-thickAg layer was deposited on the HTM layer by thermal evaporation. Theactive area of each device was about 0.15-0.28 cm².

In connection with characterizing the fabricated structures, the crystalstructures of the perovskite films were measured by X-ray diffraction(XRD, Rigaku D/Max 2200 diffractometer with Cu K_(α) radiation). Theabsorption spectra of the mesoporous and planar perovskite films werecharacterized by a UV/Vis-NIR spectrophotometer (Cary-6000i). Thephotocurrent-voltage (J-V) characteristic of perovskite CH₃NH₃PbI₃ solarcells were measured with a Keithley 2400 source meter under thesimulated AM 1.5G illumination (100 mW/cm²; Oriel Sol3A Class AAA SolarSimulator), from open circuit to short circuit with a scan rate of 0.2V/s. J-V hysteresis was observed when scanned from short circuit to opencircuit, especially for planar devices. Charge transport andrecombination properties of the mesostructured perovskite cells weremeasured by intensity-modulated photocurrent and photovoltagespectroscopies. Impedance spectroscopy (IS) was performed using aPARSTAT 2273 workstation with the frequency range of 0.1 Hz-100 kHz andthe modulation amplitude of 10 mV. The IS spectra were analyzed usingZView 2.9c software (Scribner Associates).

Results and Discussion

FIG. 6 shows perovskite films prepared from CH₃NH₃PbI₃ precursors withdifferent amounts of CH₃NH₃Cl (or MACl; as indicated) and annealed atabout 100° C. with varying duration (as indicated). FIG. 6 shows theeffect of varying annealing time (from about 1 minute to about 45minutes) at about 100° C. on the appearance of perovskite CH₃NH₃PbI₃films prepared by using the perovskite precursors with different amountsof added CH₃NH₃Cl (MACl). The perovskite precursor solution containedPbI₂, CH₃NH₃I (or MAI), and MACl with a molar ratio of 1:1:x, where xvaried from 0 to 2. The thickness of the mesoporous TiO₂ film was about650 nm. When no MACl was used, the perovskite film turned brown withinone minute of annealing at 100° C. The absorbance of this film stayedvirtually unchanged at 5 minutes and then decreases when annealed for 10minutes (FIG. 7a ). With an increasing amount of MACl added to theprecursor solution, the process that turned the perovskite filmbrown/dark brown with annealing occurred at significantly slower rates.When 2 M MACl was used, it took more than 25 minutes for the perovskitefilm to turn brown. The annealing-time-dependent absorption spectra forthese perovskite films prepared using different amounts of MACl areshown in FIGS. 7a-d . Adding MACl to the precursor solution not onlyslowed down the perovskite film darkening process but also darkened thecolor the final perovskite films (the color of the final perovskite filmchanged from brown with no MACl to dark brown with 1-2 MACl; FIG. 6).

FIGS. 8a-c depict (a) XRD patterns of CH₃NH₃PbI₃ on mesoporous TiO₂ filmas a function of the MACl amount in the precursors for differentannealing time at about 100° C.; magnified views of the XRD patterns (b)near 14 degrees for perovskite films using 0-2 M MACl with theirrespective optimum annealing time; and magnified views of the XRDpatterns (c) for the 2 M MACl sample with different annealing times(from about 1 minute to about 45 minutes). Peaks associated with theperovskite CH₃NH₃PbI₃ structure are labeled. The other peaks are eitherfrom the TiO₂/FTO substrate or from a transient intermediate structurerelated to MACl.

FIG. 8a compares the X-ray diffraction (XRD) patterns of CH₃NH₃PbI₃ onmesoporous TiO₂ film prepared from precursor solutions containingdifferent amounts of MACl. When the absorption spectrum reached aboutmaximum for each amount of added MACl (about 5 minutes for 0 M MACl, 10minutes for 0.5 M MACl, 25 minutes for 1M MACl, and 45 minutes for 2 MMACl), the perovskite samples showed the same XRD patterns forCH₃NH₃PbI₃. The strongest CH₃NH₃PbI₃ (110) peak near 14 degreesincreased slightly in intensity and became narrower as the amount ofMACl added to the precursor solution increased (FIG. 8b ). This suggestsan increase in crystallinity or a change in the preferred orientation ofthe final target perovskites with higher MACl concentration, which,without wishing to be bound by theory, is presumably caused by a slowercrystallization process at higher MACl concentration. The perovskitefilm using 2 M MACl displayed the longest time to complete a colorchange, and its XRD pattern, as a function of the annealing time (fromabout 1 minute to about 45 minutes) at ˜100° C., are shown in FIG. 8a .The main characteristic CH₃NH₃PbI₃ (110) peak near 14 degrees onlyappeared after about 25 minutes of thermal treatment. At the early stageof thermal treatment (from about 1 minute to about 10 minutes), severalintermediate peaks were present (FIG. 8c ), which all disappeared after45 minutes of thermal treatment. Table 1 shows the energy dispersiveX-ray (EDX) analysis of the perovskite halide films prepared with the 2M MACl solution, suggesting that a significant amount of chlorine isinitially incorporated into the film with an I:Cl ratio of about2.7:1.6. The amount of chloride detected in the perovskite filmdecreased rapidly from 1.6 to 0.1 during thermal treatment times rangingfrom about 1 minute to about 25 minutes. Longer annealing time (˜45minutes) led to a complete loss of chloride (within the EDX detectionlimit of 1%). The intermediate XRD peaks for the 2 M MAC1 samples withshort annealing times (from about 1 minute to about 10 minutes) areassociated with the incorporation of MACl or its variation, whichsublimes from the film with longer annealing times. The significantstructural change for the 2 M MACl sample based on XRD measurement isconsistent with the changes of its absorption spectra as a function ofannealing time (FIG. 7d ). Similar to the 2 M MACl sample, no chloridewas observed for other samples using zero molar or the other loweramounts of MACl with sufficient annealing times (Table 1). These resultssuggest that adding MACl to the CH₃NH₃PbI₃ precursor solution forms anintermediate crystal structure related to MACl, which slows down thecrystallization of CH₃NH₃PbI₃. The intermediate structure subsequentlyappears to disappear with prolonged annealing to finally yield thetargeted final perovskite halide film; e.g. CH₃NH₃PbI₃.

TABLE 1 Effect of MACl Amount (x) and Annealing Time on the Pb:I:ClRatio of Perovskite Films. x MACl (time) Pb I Cl 2 (1 min)  1  2.7(0.3^(a)) 1.6 (0.2) 2 (10 min) 1 2.6 (0.3) 0.6 (0.1) 2 (25 min) 1 2.8(0.3) 0.1 (0.1) 2 (45 min) 1 2.9 (0.3) — 1 (25 min) 1 3.0 (0.3) — 0.5(10 min)  1 3.0 (0.4) — 0 (5 min)  1 2.9 (0.4) — ^(a)The errors of theelement ratios are obtained based on the EDX detection limit of 1%.

FIGS. 9a-h show the typical scanning electron microscopy (SEM) images oftop views of the annealed perovskite films prepared from precursorsolutions with different amounts of MACl. FIGS. 9a-d correspond to theperovskite films deposited on mesoporous TiO₂ films (650-nm thickness)using from 0 M to 2 M MACl, respectively. When no MACl was used, someislands of sub-micron-sized CH₃NH₃PbI₃ were observed on the top surfaceof the mesoporous TiO₂ film. These islands either disappeared or becameless visible when MACl was used. Despite the slight difference of theperovskite appearance on the top surface of TiO₂ films, the overallappearances of these four films were similar and adopted the structureof the underlying mesoporous TiO₂ films. In contrast, adding MACl to theprecursor solution lead to dramatic changes of the morphology of theperovskite CH₃NH₃PbI₃ films deposited directly on compact TiO₂ on thefluorine-doped tin oxide (FTO) glass substrate. FIG. 9e-h correspond tothe perovskite films deposited on compact TiO₂ using from 0 M to 2 MMACl, respectively. When no MACl was used, large elongated crystalplates formed, with a significant portion of the substrate being exposedwithout CH₃NH₃PbI₃ coverage. When 0.5 M MACl was used, the large crystalplates partially disappeared and some small crystals started to form,leading to enhanced surface coverage by the perovskite halide films.When using more MACl (1 M and 2 M), the elongated large crystal platestotally disappeared and the substrate was coated with interconnected,relatively small crystals with a high surface coverage. Thesedifferences in the surface coverage of perovskite films on the planarsubstrate are likely to affect final device characteristics.

FIGS. 10a-b depict the effect of MACl on the J-V curves of (a)mesostructured and (b) planar perovskite CH₃NH₃PbI₃ solar cells. Alldevices used spiro-MeOTAD as a hole conductor and silver as a topcontact. FIG. 10a shows the effect of MACl on the currentdensity-voltage (J-V) characteristics of mesostructured perovskite solarcells based on 650-nm-thick TiO₂ mesoporous films. The cell with 0 MMACl demonstrated a short-circuit photocurrent density (J_(sc)) of˜16.91 mA/cm², open-circuit voltage (V_(oc)) of ˜0.835 V, and fillfactor (FF) of ˜0.583 to yield an efficiency (η) of ˜8.23%. The cellefficiency increased to about 9% to 10% when using MACl (0.5 M to 2 M)to adjust the crystallization process of the perovskite halide;CH₃NH₃PbI₃. The efficiency improvement resulted mainly from the higherJ_(sc) associated with stronger light absorption (FIGS. 7a-d ) due tothe use of the MACl. When CH₃NH₃PbI₃ was deposited directly on theplanar compact TiO₂, the device performance (FIG. 10b ) showed a similartrend with MACl compared to the mesostructured perovskite solar cells.The degree of performance improvement by MACl for planar devices wasgreater than the performance improvement for the mesostructured devices.The J_(sc) increased from about 6 mA/cm² to about 18 mA/cm² when theamount of MACl was changed from 0 M to 0.5 M. When 1 M to 2 M MACl wasused, the J_(sc) increased to >20 mA/cm². The V_(oc) increased fromabout 0.8 V for 0 M MACl to >1 V for 0.5 M to 2 M MACl. The FF increasedfrom about 0.41 to a range of 0.57-0.61 for the same changes in MAClconcentrations. As a result, the overall efficiency was improved fromabout 2% (0 M MACl) to ˜11% (0.5 M MACl) to ˜12% (1 M to 2 M MACl). Thedevice parameters (J_(sc), V_(oc), FF, and η) for all these mesoporousand planar cells are summarized in Table 2.

TABLE 2 Effect of MACl Amount (x) on Short-Circuit Photocurrent DensityJ_(sc), Open-Circuit Voltage V_(oc), Fill Factor FF, and ConversionEfficiency η of Solid-State Mesostructured (Meso) and Planar PerovskiteCH₃NH₃PbI₃ Solar Cells. The mean values and standard deviations of thePV parameters from 12-20 cells for each type of devices are given inparentheses. Cell Type (x) J_(sc) (mA/cm²) V_(oc) (V) FF η (%) Meso (0)16.91 0.835 0.583  8.23 (16.96 ± 0.64) (0.826 ± 0.013) (0.545 ± 0.032)(7.64 ± 0.64) Meso (0.5) 18.55 0.845 0.582  9.12 (17.91 ± 0.67) (0.836 ±0.016) (0.552 ± 0.039) (8.25 ± 0.54) Meso (1) 19.31 0.833 0.595  9.57(19.44 ± 0.61) (0.824 ± 0.019) (0.565 ± 0.023) (9.03 ± 0.33) Meso (2)19.48 0.829 0.625 10.09 (19.38 ± 0.50) (0.823 ± 0.016) (0.597 ± 0.020)(9.52 ± 0.37) Planar (0)  5.55 0.813 0.413  1.86  (4.75 ± 0.82) (0.735 ±0.060) (0.375 ± 0.035) (1.34 ± 0.39) Planar (0.5) 17.90 1.004 0.60710.91 (17.38 ± 0.66) (0.974 ± 0.022) (0.561 ± 0.038) (9.50 ± 0.85)Planar (1) 20.85 1.016 0.566 11.99 (20.08 ± 0.76) (1.019 ± 0.029) (0.515± 0.048) (10.51 ± 0.92)  Planar (2) 20.36 1.023 0.581 12.10 (19.84 ±0.63) (1.013 ± 0.042) (0.540 ± 0.029) (10.85 ± 0.79) 

FIGS. 11a-b illustrate the effect of MACl on the recombinationresistance (R_(rec)) as a function of voltage for (a) mesostructured and(b) planar perovskite CH₃NH₃PbI₃ solar cells. Impedance spectroscopy wasused to study the effect of using MACl on the recombination resistance(R_(rec)) for mesostructured and planar perovskite CH₃NH₃PbI₃ solarcells. FIG. 11a shows the R_(rec) values as a function of voltage formesostructured perovskite solar cells. The R_(rec) for allmesostructured perovskite solar cells varied signifincantly with thebias voltage, following an approximately exponential decrease withvoltage. All mesostructured perovskite cells exhibited essentially thesame voltage dependence of R_(rec), suggesting that using MACl as asecond alkyl ammonium halide in the solution has minimal effect on therecombination kinetics in mesostructured cells. In contrast, a strongMACl effect was evident on the voltage dependence of R_(rec) for planarperovskite cells (FIG. 11b .) The R_(rec) values for the planar devicenot using MACl were generally from about 1 to 2 orders of magnitudelower than those for the planar devices using MACl to assist thecrystallization of CH₃NH₃PbI₃. Thus, the recombination rate for planarsamples prepared without MACl was much faster than that for planarsamples prepared with MACl. The observed difference in R_(rec) for theseplanar devices is consistent with their markedly different dark J-Vcharacteristics (FIGS. 12a-b ). The onset voltage of the dark currentshifted from about 500 mV to over 700 mV when MACl was used as a secondalkyl ammonium halide in the solution. Both R_(rec) and dark J-V resultswere consistent with the influence of MACl on the morphologies of planarperovskite CH₃NH₃PbI₃ films grown directly on the compact TiO₂ layer(FIGS. 9e-h ). When no MACl was used, a significant portion of thesubstrate was exposed without CH₃NH₃PbI₃ coverage, which may lead toenhanced recombination and decreased solar cell performance.

To help understand the role of MACl on the perovskite film formationusing the disclosed solution mixtures (i.e., a mixture of PbI₂, MAI, anda second alkyl ammonium halide with different molar ratios), perovskitefilms prepared from a solution containing only PbI₂ and MACl in theabsence of MAI were evaluated. These films will be referred to herein asPbI₂-MACl. FIG. 13 shows the effect of thermal treatment time (at about100° C.) on the XRD patterns of the PbI₂-MACl films prepared from thestoichiometric solution of PbI₂ and MACl with the same Pb concentrationas used in the disclosed MACl-added solutions for making perovskitehalides films; CH₃NH₃PbI₃. After about 1 minute of thermal treatment,the PbI₂—MACl film displayed the characteristic perovskite (110) peaknear 14 degrees. There were two additional new XRD peaks at about 12.6and 15.5 degrees, respectively. In contrast, the perovskite film usingMACl-added CH₃NH₃PbI₃ precursor exhibited different XRD patterns duringthe early stage of annealing (FIGS. 8a-c ). The peak near 12.6 degreesmay be attributed to the formation of PbI₂, whereas the peak near 15.5degrees may be attributed to the formation of CH₃NH₃PbCl₃. Withincreased thermal treatment time, the intensities of both the 14- and15.5-degree perovskite peaks decreased. In contrast, the 12.6-degreePbI₂ peak increased with thermal treatment time. With thermal treatmenttimes longer than about 50 minutes, the two perovskite peaks near 14 and15.5 degrees disappear completely. These XRD results show that thePbI₂-MACl film is not stable and decomposes to primarily PbI₂ withannealing. Consistent with the XRD results, the absorption spectrum ofthe PbI₂-MACl film also undergoes significant changes with thermaltreatment time (FIG. 15). The initial absorption spectrum of thePbI₂-MACl film was similar to that of CH₃NH₃PbI₃ with a brown color. Thefilm turned yellowish after >20 minutes of thermal treatment, and itsabsorption spectrum become dominated by the absorption of PbI₂, which isconsistent with the XRD results. Devices based on the PbI₂-MACl filmswith from 5 minutes to about 10 minutes of thermal treatment only show2-3% efficiencies. These results suggest that direct reaction betweenPbI₂ and MACl (in the absence of MAI) may not lead to the formation ofpure, high quality CH₃NH₃PbI₃ perovskite films. Furthermore, the maximummolar ratio of MACl:PbI₂ in the PbI₂-MACl precursor solutions was 1:1.Using a larger molar ratio for MACl:PbI₂ did not lead to a clear,fully-dissolved starting solution. In contrast, the solutions describedherein containing PbI₂, MAI, and from 0.5 M to 2 M MACl were clear andstable in ambient conditions. These results show that the exactprecursor composition is critical to the formation of perovskite films.Thus, the success in preparing CH₃NH₃PbI₃ film in this study cannot beassociated with PbI₂-MACl in the absence of MAI. Without wishing to bebound to by theory, it is believed that the CH₃NH₃PbI₃ films, withMACl-added to the MAI and PbI₂ starting solution, are formed through apossible intermediate, MAI.PbI₂.MACl, with an unknown crystallinestructure. The additive MACl may act as a “sacrificial” agent to formthis intermediate and then slowly decompose, volatilize, and/or sublimeduring the annealing process.

Charge transport and recombination properties in mesostructuredperovskite CH₃NH₃PbI₃ solar cells were studied by IMPS and IMVS. FIG.14a shows the effect of using MACl on the diffusion coefficient (D) as afunction of photoelectron density (n). All cells exhibited essentiallythe same power-law dependence (D∝n^(1/α-1), with α being a disorderparameter) that is attributable to the electrons undergoing multipletrapping and detrapping through the mesoporous electrode film. There wasno obvious difference of the D values for mesostructured perovskitecells using different amounts of MACl in the CH₃NH₃PbI₃ precursorsolution, suggesting that using MACl does not affect the trapdistribution on the TiO₂ surface. Similarly, no significant differenceis observed for the recombination lifetime as a function of voltage formesostructured cells prepared using different amounts of MACl (FIG. 14b).

FIGS. 16a-c provide (a) an SEM image, (b) XRD patterns, and (c) EDXanalysis of the mixed halide CH₃NH₃PbI₃-xClx prepared from the startingsolution containing MAI and PbCl₂ (at about a 3:1 molar ratio). FIG. 16ashows the SEM image of the CH₃NH₃PbI_(3-x)Cl_(x) film thermally treatedat about 100° C. for ˜45 minutes. The overall film morphology wassimilar to the CH₃NH₃PbI₃ films prepared using the CH₃NH₃PbI₃ precursorswith the addition of from about 1 M to about 2 M MACl (FIGS. 9g and 9h). The crystal structure evolution of the CH₃NH₃PbI_(3-x)Cl_(x) film wasexamined by the thermal treatment time dependence of the XRD patterns.With ˜1 minute of thermal treatment at 100° C., the film exhibited threeXRD peaks near 12.6, 14, and 15.5 degrees, which were similar to the XRDpatterns of the PbI₂-MACl films (FIG. 13) during the early stage ofannealing. Again, the perovskite film using MACl added to the MAI andPbI₂ starting solution exhibited different XRD patterns during theinitial thermal treatment stage, implying the existence of differentcrystal structures formed during the early stage of thermal treatment.With increasing thermal treatment time, to about 45 minutes, only theperovskite peak near 14 degrees is left for the CH₃NH₃PbI_(3-x)Cl_(x)film, similar to the XRD patterns for CH₃NH₃PbI₃ shown in FIGS. 8a-c .Consistent with this observation, the EDX analysis (FIG. 16c ) of theCH₃NH₃PbI_(3-x)Cl_(x) film shows that with increasing thermal treatmenttime, the Cl:I ratio decreased from about 1.6:2.7 at 1 about minute to0.5:2.8 at about 20 minutes. The CH₃NH₃PbI_(3-x)Cl_(x) film thermallytreated for ˜45 minutes did not show any detectable trace of chloride.

FIG. 17 illustrates recombination resistance R_(rec) as a function ofvoltage for a planar perovskite solar cell based on the mixed halideCH₃NH₃PbI_(3-x)Cl_(x) prepared from the starting solution containing MAIand PbCl₂ (3:1 molar ratio). The R_(rec) values for the planarCH₃NH₃PbI₃ cells prepared from the CH₃NH₃PbI₃ precursor with 2-MACladditive are also plotted for comparison. The J-V curve for a planarCH₃NH₃PbI_(3-x)Cl_(x) perovskite cell is shown in FIG. 18. The cellefficiency was 11.86% with a J_(sc) of ˜20.57 mA/cm², V_(oc) of 0.995 V,and FF of ˜0.579. The performance of the planar CH₃NH₃PbI₃-xClxperovskite cell was similar to that of the planar CH₃NH₃PbI₃ cellsprepared from the starting solutions containing from 1 M to 2 M amountsof MACl additives, as previously described. Referring again to FIG. 17,the R_(rec) values for the planar CH₃NH₃PbI₃ cells prepared with 2-MACladditive are also plotted for comparison. It is apparent that these twosamples follow almost identical voltage dependence of the recombinationresistances. Taken together the results of SEM, XRD, J-V, and impedancespectroscopy, it is reasonable to believe that the CH₃NH₃PbI_(3-x)Cl_(x)perovskite film prepared from the 3MAI-PbCl₂ precursor is similar to theCH₃NH₃PbI₃ perovskite film prepared from the MACl-added to the solutioncontaining MAI and PbI₂. Despite these similarities, significantdifferences between these two approaches exist. First, the crystalstructures of the perovskite films prepared with these two approachesduring the early stage of thermal treatment are different (FIGS. 8a-cand 16a-c ), indicating that the exact precursor composition is criticalto the process of perovskite formation. Second, it was also found thatthe ratio of MAI:PbCl₂ in the starting solution for preparingCH₃NH₃PbI_(3-x)Cl_(x) should not be less than 3; otherwise, PbCl₂ cannotbe fully dissolved. This may explain why the CH₃NH₃PbI_(3-x)Cl_(x)precursor has a molar ratio of 3:1 for MAI:PbCl₂. In contrast, astarting solution utilizing wide ranging amounts of MACl added to MAIand PbI₂, enhanced the ability to control the kinetics of the perovskitecrystallization process. Third, this example shows that the 3MAI-PbCl₂precursor solution turned from green-yellow to brown-yellow after oneweek of storage in air. In contrast, the PbI₂-MAI-2MACl startingsolution (with the same Pb concentration as in the 3MAI-PbCl₂ precursorsolution) stays clear with a green-yellow color for more than threemonths when stored in air.

In summary, the present disclosure provides a one-step solution approachto prepare perovskite halide films (CH₃NH₃PbI₃) on a mesoporous TiO₂film or on a planar, compact TiO₂ layer on FTO. In this Example,CH₃NH₃Cl (or MACl) was added to a standard equimolar solution of CH₃NH₃Iand PbI₂ to favorably adjust the crystallization process for the finaltarget perovskite halide film; CH₃NH₃PbI₃. Depending on the amount ofMACl used in the precursor solution, the optimum crystallization timefor forming pure CH₃NH₃PbI₃ with the strongest absorption varied from afew minutes to several tens of minutes. The use of MACl not only led toenhanced absorption of CH₃NH₃PbI₃, but also to significantly improvedcoverage of CH₃NH₃PbI₃ on a planar substrate. Compared to a typical,standard one-step solution approach for producing CH₃NH₃PbI₃ films, theadditional use of an alkyl ammonium halide such as MACl improves theperformance of perovskite solar cells. For the mesostructured devicearchitecture, the efficiency was enhanced from about 8% to 10%, whereasfor the planar cell structure, the efficiency was improved from about 2%to 12%. The recombination resistance for planar cells was reduced by 1-2orders of magnitude by using MACl. The significant performanceimprovement for planar perovskite cells is attributed primarily to theimproved morphology of the perovskite films resulting from theadditional use of MACl to control the crystallization process forforming the final target perovskite halide film, CH₃NH₃PbI₃.

Example 2: One-Step Method

FIGS. 19a-c depict typical SEM images of CH₃NH PbI₂Br films deposited onc-TiO₂/FTO substrates from various precursor solutions: (a) PbI₂+MABr;(b) PbI₂+MABr+MACl; and (c) PbI₂+MABr+2MACl. FIGS. 20a-c illustrate theeffect of annealing time (as indicated) at 125° C. on the XRD patternsof CH₃NH₃PbI₂Br films deposited on c-TiO₂/FTO substrate from variousprecursor solutions: (a) PbI₂+MABr; (b) PbI₂+MABr+MACl; and (c)PbI₂+MABr+2MACl. FIG. 21 illustrates photocurrent density-photovoltage(J-V) curves of perovskite solar cells prepared from different precursorcompositions (i.e., PbI₂+MABr or 0 MACI; PbI₂+MABr+MACl or 1 MACl; andPbI₂+MABr+2MACl or 2MACl).

This one-step solution approach provided by the present disclosurecomprises introducing CH₃NH₃Cl (or MACl) to the standard CH₃NH₃PbI₂Brprecursor solution to prepare perovskite CH₃NH₃PbI₂Br films. It has beenfound that the use of CH₃NH₃Cl alters the crystallization process forforming CH₃NH₃PbI₂Br. The amount of CH₃NH₃Cl in the perovskite precursorsolution strongly affects the film morphology (FIGS. 19a-c ), crystalstructure formation (FIGS. 20a-c ), and device characteristics (FIG.21).

Example 3: Three-Step Method

This example illustrates a three-step sequential deposition method forpreparing PbI₂-free CH₃NH₃PbI₃ films. Using typical, known two-stepapproaches, it is often difficult to completely convert PbI₂ into finaltarget perovskite halide film, e.g. CH₃NH₃PbI₃, without causingsignificant degradation of the perovskite film. In contrast, thedisclosed three-step sequential solution deposition approach provides athermally unstable stoichiometric PbI₂—CH₃NH₃C₁ (PbI₂.MACl) transitory,intermediate film that is first deposited on the mesoporous TiO₂substrate, followed by thermal decomposition to form a PbI₂ film. Usingthis disclosed method, the PbI₂ film can be rapidly converted intoCH₃NH₃PbI₃ without any PbI₂ residue by using a MAI IPA solution at roomtemperature, which ultimately leads to improved device performance.

Experimental Setup and Device Fabrication

Preparation of material for fabricating the disclosed structures, andfor fabricating conventional devices used for purposes of comparison,used to quantify the improved performance of the disclosed structuresproceeded as follows. MAI and MACl were synthesized by reactingmethylamine (MA) with HI and HCl, respectively, followed bypurification. A patterned fluorine-doped tin oxide (FTO) was firstdeposited with a blocking layer by spray pyrolysis at ˜450° C. using˜0.2 M Ti(IV) bis(ethyl acetoacetate)-diisopropoxide 1-butanol solution,followed by ˜450° C. annealing in air for one hour. A 500-nm-thickmesoporous TiO₂ film was then screen coated onto FTO followed by ˜500°C. annealing in air for 30 minutes. The TiO₂ film was then treated in 40mM TiCl₄ aqueous solution at ˜65° C. for about 30 minutes. TheseTiCl₄-treated TiO₂ films were then sequentially rinsed by deionized (DI)water, blown dry in air, and finally annealed at about 500° C. for about30 minutes.

“Standard” two-step deposition of perovskites proceeded as follows. From0.4 M to about 1.0 M PbI₂ (99.99%) DMF solution was spin coated onto amesoporous TiO₂ film at 3000 rpm for about 20 seconds. The depositedPbI₂ film was then thermally treated at about 70° C. for about 5minutes. The PbI₂ film was cooled to room temperature and then immersedinto a 10 mg MAI/mL isopropyl alcohol (IPA) solution for different times(from 2 minutes to about 60 minutes), followed by rinsing with IPA,blown dry with N₂, and then thermally treated at 70° C. for about 5minutes.

Three-step methods for the formation perovskites halides, performedpursuant to methods provided by the present disclosure, as follows. Afirst solution in DMF of 1.0 M equal molar ratio PbI₂ and MACl was firstspin coated onto the mesoporous TiO₂ film at 3000 rpm for about 20seconds. The applied solution was then thermally treated at about 130°C. for about 30 minutes, resulting in the formation of a yellow PbI₂film, which was then cooled to room temperature. The yellow PbI₂ filmwas then immersed into a 10 mg MAI/mL IPA solution for about 30 seconds,followed by rinsing with IPA, blown dry by N₂, and then thermallytreated at ˜70° C. for about 5 minutes.

In connection with device preparation, the perovskite-depositedelectrodes were first coated with a layer of hole transport material(HTM) by spin coating at 4,000 rpm for 20 seconds using 0.1 Mspiro-MeOTAD, 0.035 M bis(trifluoromethane)sulfonimide lithium salt(Li-TFSi), and 0.12 M 4-tert-butylpyridine (tBP) inchlorobenzene/acetonitrile (10:1, v/v) solution. A 150-nm-thick Agcontact layer was then deposited by thermal evaporation.

The crystalline structures of the perovskite films were measured byX-ray diffraction (XRD, Bruker D8 ADVANCE with Cu Kα radiation). Theabsorption spectra of the planar perovskite films were characterized byan ultraviolet-visible (UV-vis) spectrophotometer (Cary-60). Thephotocurrent density-voltage (J-V) characteristic of perovskite solarcells was measured with a Keithley 2400 source meter under simulated AM1.5G illumination (100 mW/cm2; Oriel Sol3A Class AAA Solar Simulator).

Results and Discussion

In a standard two-step method, a 1.0 M PbI₂ precursor solution isnormally used to obtain sufficient deposition of CH₃NH₃PbI₃ to absorblight. In this Example, the time evolution of the conversion of PbI₂into the final target perovskite halide film, CH₃NH₃PbI₃, was examinedby using UV-vis and XRD measurements. FIGS. 22a-b show the effect of MAIcontacting times on changes of the XRD patterns. When the PbI₂ film wascontacted with MAI for about 2 minutes, there was an incompleteconversion as evidenced by the existence of both the PbI₂ (near 12.6°)and CH₃NH₃PbI₃ (near 14°) diffraction peaks with similar peakintensities. The intensity of the CH₃NH₃PbI₃ peak increased, whereas thePbI₂ peak intensity decreased with longer MAI contact times (from 10minutes to 60 minutes). Even with 60 minutes of contact time, there wasstill a noticeable amount of PbI₂ in the film as indicated by thecharacteristic PbI₂ diffraction peak. However, the intensity of the PbI₂peak was much smaller and almost negligible compared to the CH₃NH₃PbI₃peak when the contact time was more than 30 minutes.

During the conversion process to the final perovskite film, the initialyellow PbI₂ film slowly changed color to brown or dark brown, dependingon the contact time in the second MAI-containing solution. FIG. 22bshows the UV-vis absorption spectra of the resulting films as a functionof the contact time. Although the XRD measurement indicates asignificant fraction of PbI₂ existed after 2 minutes of contact time,the UV-vis spectrum of this sample showed a typical CH₃NH₃PbI₃absorbance as shown in FIG. 22b . The direct absorbance by the residuePbI₂ is likely hidden by the absorption spectrum from the partiallyconverted CH₃NH₃PbI₃ within the film. Because the regular IPA solutioncan dissolve CH₃NH₃PbI₃, longer contact times eventually led to asignificant drop of absorption across the entire wavelength range,despite more conversion of PbI₂ to the final target perovskite film,CH₃NH₃PbI₃, as suggested by the XRD results. Therefore, the samplecontacted for about 60 minutes with the second solution only displayedabout half the absorbance compared to the sample contacted with thesecond solution for about 2 minutes.

The impact of the MAI-containing contact time on the photovoltaicparameters of the perovskite solar cells are presented in Table 3. Whenthe contact time was increased from 2 minutes to 60 minutes, theshort-circuit photocurrent density first increased and then decreased,with a peak value (14.84 mA/cm2) reached at 30 minutes contact time,which reflects the balance between perovskite conversion and dissolutionduring the second contacting step, as shown in FIGS. 22a-b . Fill factorof these devices increased significantly with the increased contacttimes, from 0.346 at 2 minutes to 0.455 at 60 minutes. The overallconversion efficiency essentially followed the trend of photocurrent,increasing from ˜2.55% at 2 minutes to ˜5.75% at 30 minutes, and thendecreases to ˜4.59% at 60 minutes. These results indicate that it isimportant to have more complete conversion of PbI₂ to the final targetperovskite halide film, CH₃NH₃PbI₃, without significant dissolution ofCH₃NH₃PbI₃ in order to reach reasonable device performance.

TABLE 3 Effect of the MAI dipping time on the perovskite solar cellphotovoltaic parameters: short-circuit photocurrent density, J_(sc);open-circuit voltage, V_(oc); fill factor, FF; and conversionefficiency, η. Dipping Time J_(sc) V_(oc) η [min] [mA/cm²] [V] FF [%] 28.58 0.859 0.346 2.55 10 11.41 0.837 0.403 3.85 30 14.84 0.899 0.4315.75 60 11.07 0.911 0.455 4.59

FIGS. 23a-b illustrate the effect of PbI₂ concentration (from 0.4 M to1.0 M) on the (a) XRD patterns and (b) UV-vis absorption spectra of theperovskite films deposited on mesoporous TiO₂ films after contacting thePbI₂ films with a 10 mg MAI/mL IPA solution for about 10 minutes. Theimpact of varying PbI₂ concentration on the PbI₂-to-CH₃NH₃PbI₃conversion using a typical two-step method was also analyzed. Theconcentration of the PbI₂ precursor solution was changed from 0.4 M to1.0 M. To avoid the dissolution of CH₃NH₃PbI₃ in the IPA solution, thedipping time of PbI₂ film in the MAI IPA solution was fixed at about 10minutes for all samples. FIG. 23a shows the XRD measurements of theperovskite films using different PbI₂ concentration with a ˜10 minutecontact time during the second contacting step with the second solution.The intensity of the characteristic PbI₂ diffraction peak decreasedsignificantly when the PbI₂ concentration was reduced from 1.0 M to 0.8M. The PbI₂ peak disappeared from the diffraction pattern when the PbI₂concentration was further reduced to 0.6 M and 0.4 M, suggesting acomplete conversion of PbI₂ to CH₃NH₃PbI₃ using a low-concentration PbI₂first solution, with ˜10 minute contact time in the second MAI IPAsolution. Without wishing to be bound to theory, it is believed thatlower concentration PbI₂ first solutions (e.g., from 0.4 M to 0.6 M) canlead to less infiltration of PbI₂ into the mesoporous TiO₂ films thanthe higher concentration first solutions (e.g., from 0.8 M to about 1.0M PbI₂). A lower degree of pore filing of PbI₂ may promote theconversion of PbI₂ into CH₃NH₃PbI₃ by facilitating the intercalation ofMAI into PbI₂; but it may also result in lower absorbance due to lessCH₃NH₃PbI₃ deposition. FIG. 23b compares the effect of PbI₂concentration on the UV-vis absorption spectra of the perovskite films.A higher PbI₂ concentration in the first solution for the firstcontacting step, generally led to stronger absorbance. The perovskitefilm based on a 0.4 M PbI₂ first solution displayed only about half theabsorbance of the perovskite film based on a 1.0 M PbI₂ first solution.

Table 4 summarizes the photovoltaic parameters (J_(sc), V_(oc), FF andr) of the solar cells based on the CH₃NH₃PbI₃ films prepared usingdifferent concentrations of PbI₂ in the first solution of the two-stepmethod, with 10 minute contact times in the second MAI solution duringthe second contacting step. The 0.8-M sample exhibited a lowphotocurrent density similar to the 1.0-M sample, which may beattributed to the residue PbI₂ observed by XRD. In contrast, the 0.6 Msample without any PbI₂ residue exhibited a reasonable photocurrentdensity despite its lower absorption than the 1.0 M sample (FIG. 22b ).However, the lowest J_(sc) value was observed for the 0.4 M sample, forwhich the poor light absorption clearly limits the device performance.As a balance for optimizing both the light absorption and conversion ofPbI₂ to CH₃NH₃PbI₃, the best device performance efficiency was obtainedfor the 0.6 M sample with a cell efficiency of about 7.23%. Thus, itappears that isopropanol may not be suitable for a standard two-stepmethod because of the dissolution of CH₃NH₃PbI₃ dissolution and thedifficulty of obtaining a PbI₂-free CH₃NH₃PbI₃ film with sufficientabsorption.

TABLE 4 Effect of PbI₂ concentration (0.4-1.0M) on the perovskite solarcell photovoltaic parameters. The second-step MAI dipping time is 10 minfor all samples. Concentration J_(sc) Voc η [M] [mA/cm²] [V] FF [%] 0.410.27 0.842 0.609 5.27 0.6 14.79 0.836 0.585 7.23 0.8 11.73 0.830 0.555.35 1.0 11.40 0.837 0.403 3.85

In order to overcome the deficiencies of the two-step deposition, thethree-step sequential deposition method, per embodiments of the presentinvention, was investigated. In the three-step method, a first solutionof PbI₂ and MACl (in equimolar concentration) was first applied onto amesoporous TiO₂ film. Upon thermal treatment at ˜130° C. for about 1minute, the resulting film exhibited a light brown color with an unknownXRD pattern. Its XRD pattern and UV-vis absorption spectra are shown inFIGS. 24a and 24b (Step 1), respectively. The absorption spectra weresimilar to those of CH₃NH₃PbI₃; however, their XRD patterns were verydifferent. This unknown compound (denoted as PbI₂+MACl) was likely amixture of CH₃NH₃PbCl₃, CH₃NH₃PbI₃, and some unknown structure. Whenthermally treated at 130° C. in air, the brown film slowly turned to ayellow color. After thermally treating at 130° C. for 30 min, thePbI₂+MACl film was thermally decomposed to PbI₂, as suggested by itsUV-vis spectra and XRD pattern (FIGS. 24a-b , Step 2). No traceablechlorine content could be found in this film by the energy dispersiveX-ray (EDX) analysis.

The PbI₂ film formed from thermal treatment turned dark brown almostimmediately after contacting with the second MAI-containing solution(with regular IPA). It took only about 30 seconds for the yellow PbI₂film to convert into a dark-brown CH₃NH₃PbI₃ film (FIG. 24a , step 3)without any traceable PbI₂ evident from its XRD pattern (FIG. 24b , step3). The perovskite halide target film, CH₃NH₃PbI₃, prepared from theinitial 1.0 M PbI₂.MACl precursor by the three-step method exhibitedstrong absorption (especially in the long-wavelength region) similar tothe film prepared from the regular two-step approach using a 1.0 M PbI₂first solution.

FIG. 25 is a graphical illustration of a J-V characteristic of aperovskite solar cell based on the CH₃NH₃PbI₃ film prepared from thedisclosed three-step method. The cell efficiency was about 10.11%, witha J_(sc) of 18.64 mA/cm2, V_(oc) of ˜0.868 V, and FF of ˜0.625. Bothforward and backward J-V scans are shown in FIG. 25. The hysteresis wasabout 1%, and thus, can be considered negligible.

The above results demonstrate that the three-step method provides apromising way to address the challenges associated with the PbI₂conversion for a standard two-step method. Without wishing to be limitedby theory, it is believed that the quick PbI₂-to-CH₃NH₃PbI₃ conversionkinetics observed results from the higher exposed surface of the PbI₂film prepared through the thermal treatment step of the three-stepdeposition method. The formation of pores resulting from the release ofMACl in the thermal treatment step is expected to form a PbI₂ film witha larger exposed surface area than the PbI₂ film deposited directly fromthe PbI₂ precursor. The higher exposed surface of the PbI₂ film mayfacilitate the intercalation reaction between MAI with PbI₂ during thecontacting step with the second solution, leading to a completeconversion of PbI₂ within about 30 seconds. With such a short contacttime, the issue of CH₃NH₃PbI₃ dissolution in the IPA solution may beeffectively mitigated.

For a typical two-step method, with a high-concentration (1.0 M) PbI₂first solution, the conversion of the PbI₂ film to the final targetperovskite halide, CH₃NH₃PbI₃, required a long conversion time in theMAI IPA second solution. The long exposure to the second IPA solutioncaused the dissolution of the converted CH₃NH₃PbI₃ perovskite film,leading to reduced photocurrent generation, and consequently, poordevice performance. On the other hand, a short exposure of the PbI₂ filmto the second MAI IPA solution resulted in only a partial conversion ofthe film with significant PbI₂ residue, which also limited the deviceperformance. Using a less concentrated first PbI₂ solution (e.g., 0.4 M)showed complete conversion of the PbI₂ film with a fixed contact time ofabout 10 minutes. However, the poorer light absorption for the finalperovskite film prepared with a low concentration of PbI₂ in the firstsolution also limited the final device performance. To address thisdilemma, the present disclosure provides a three-step solution method toprepare CH₃NH₃PbI₃ perovskite halide films by using a secondMAI-containing solution with regular (non-anhydrous) IPA. In comparisonto the typical two-step approach using similar processing conditions,the disclosed three-step method enables the formation of a PbI₂ filmthrough the thermal decomposition of an intermediate and transitoryPbI₂+MACl film. This intermediate film facilitates a rapid conversion ofthe subsequently formed PbI₂ film to the final target perovskite halidefilm, CH₃NH₃PbI₃, without any detectable residual PbI₂ in the perovskitefilm, leading to improved device performance. Thus, the disclosedthree-step solution method using regular IPA represents a promisingalternative deposition method for preparing low-cost, high-efficiencyperovskite solar cells.

Example 4: Modified Three Step Method

In this Example, the three-step method provides a facilemorphology-controllable sequential deposition of a planar CH₃NH₃PbI₃film by stepping through a transitory intermediate PbI₂.xMAI (x:0.1-0.3) precursor film. Adding a small amount of MAI to the PbI₂ firstsolution, may lead to about a 10-fold faster complete formation of thefinal target perovskite halide film, CH₃NH₃PbI₃, without any PbI₂residue after the contacting step with the MAI-containing secondsolution, resulting in a much improved device performance with higherreproducibility, especially when using about 0.1 M to about 0.2M MAI inthe second solution. The best cell efficiency recorded was 16.76% usingthis modified PbI₂.0.15MAI first solution with the maximum power outputstabilized at about 16.5% under one-sun illumination. This demonstratesthat the morphologies of PbI₂.xMAI intermediate films and thecorresponding final target perovskite halide films, CH₃NH₃PbI₃, can beadjusted by tuning the relative amount of MAI.

FIG. 26a shows the ultraviolet-visible (UV-vis) absorption spectraevolution of a pure PbI₂ film after dipping in a second solution of 10mg/mL MAI isopropanol (IPA) for contact times up to about 30 minutes.The absorption shoulder near 750 nm for perovskite CH₃NH₃PbI₃ increasedslowly with the MAI dipping time up to 20 min. The absorption spectrumincreased very little after 20 minutes of contacting, which indicatesthat most PbI₂ had been converted to CH₃NH₃PbI₃. The baseline of theabsorption spectra also increased simultaneously with longer dippingtime, reflecting the stronger light scattering effect associated with arougher perovskite morphology. Although the absorption measurementssuggest that the CH₃NH₃PbI₃ perovskite film grown with 6 minutes dippingexhibited strong CH₃NH₃PbI₃ absorbance spectrum, the X-ray diffraction(XRD) pattern of the same sample (FIG. 26b ) still exhibited thecharacteristic peaks for PbI₂ along with the CH₃NH₃PbI₃ diffractionpeaks, which is an indication of incomplete conversion of PbI₂ toCH₃NH₃PbI₃. The intensity of PbI₂ diffraction peak was substantiallyreduced with longer contacting times up to 20 minutes. The PbI₂ peakdisappeared completely when the contact time was further increased to 30minutes. The typical scanning electron microscopy (SEM) images of planarPbI₂ and CH₃NH₃PbI₃ (20-min dipping/conversion) films are compared inFIGS. 26c and 26d , respectively. The planar PbI₂ film appeared porouswith a coarse top surface, which may be caused by the shrinkage of theprecursor film during the drying process. The resulting CH₃NH₃PbI₃ filmwas also full of tiny pinholes and its surface was covered with roughperovskite nanocrystals; this appearance is similar to that of theperovskite capping layer grown on a mesoporous substrate using two-stepsequential deposition. The increased roughness of the CH₃NH₃PbI₃ filmcompared to the initial PbI₂ film may be attributed to space expansionduring the second step of MAI intercalation reaction with PbI₂.

Taken together, the results of XRD and absorption measurements in FIGS.26a and 26b illustrate the deficiencies associated with planarCH₃NH₃PbI₃ thin films prepared from PbI₂ using the standard two-stepsequential solution deposition method. The effect of varying MAI dippingtime on the device characteristics is shown in FIG. 27 and thecorresponding photovoltaic parameters are listed Table 5. The cellefficiency increased significantly with longer contact time from 3.145%at 0.5 min to 6.105% at 2 min to 9.301% at 6 min, which was consistentwith the more complete conversion of PbI₂ to CH₃NH₃PbI₃. However, whenthe contact time was increased to 20 min, the device performance droppedin all parameters to an overall efficiency of only 1.290%, despite thefact that the CH₃NH₃PbI₃ film with 20 min contact time showed a reducedamount of residue PbI₂ and also had stronger light absorption than theCH₃NH₃PbI₃ film with 6 minute contact times. Thus, simply lengtheningthe conversion time to complete the PbI₂-to-CH₃NH₃PbI₃ conversionpresents a challenge for the typical two-step sequential solutionprocessing. The long contacting/reaction duration caused damage to theCH₃NH₃PbI₃ layer resulting from the back extraction of MAI fromCH₃NH₃PbI₃, thus deteriorating the device performance.

TABLE 5 Device parameters of planar CH₃NH₃PbI₃ solar cells prepared fromPbI₂ precursor films. PbI₂ in J_(sc) V_(oc) η MAI time (mA/cm²) (V) FF(%) 0.5 min  6.539 0.979 0.491 3.145 2 min 8.847 0.969 0.712 6.105 6 min15.515 0.961 0.624 9.301 20 min  4.512 0.489 0.584 1.290

To address the challenges associated with the incomplete conversion ofPbI₂ and rough perovskite surface discussed above and associated withthe known two-step approach, the modified three-step deposition processprovided by the present disclosure has been developed. The disclosedmodified three-step method utilizes an alkyl ammonium halide incombination with the typical perovskite precursor, a mixture of PbI₂ andx alkyl ammonium halide with x varying from 0.1-0.3, during a firstcontacting step to deposit a resulting precursor film denoted as PbI₂.xalkyl ammonium halide. Even in the presence of the alkyl ammoniumhalide, the deposited PbI₂.x alkyl ammonium halide films still lookedyellowish without any indication of the formation of perovskiteCH₃NH₃PbI₃.

In this Example, the alkyl ammonium halide was methyl ammonium iodide(MAI). FIG. 28a shows the UV-vis absorption spectra of the PbI₂.xMAIprecursor films. All of these films exhibited an absorption onset near510 nm, which is characteristic for PbI₂. The sharp absorption edgeassociated with PbI₂ became less clear with increasing MAI. At x=0.3,the baseline of the absorption of the precursor film increasedsubstantially, indicating an enhanced light scattering effect. FIG. 28bcompares the XRD patterns of PbI₂.xMAI (x: 0-0.3) precursor films. Inthe absence of MAI (x: 0), the PbI₂ film displayed a strongcharacteristic XRD peak at about 12.5°. The intensity of PbI₂ peaksdecreased significantly with the increase of the MAI amount used in thefirst-step precursor solution. The addition of xMAI into PbI₂ during thefirst deposition step did not result in any detectable CH₃NH₃PbI₃diffraction peaks, such as the main (110) peak near 14°. This isconsistent with the yellow appearance and the corresponding absorptionspectra. The intensity of the main PbI₂ diffraction peak (˜12.5°)decreased substantially with increased amounts of MAI in the precursor.This observation suggests that a partial incorporation of MAI in thePbI₂ film decreased the PbI₂ crystallinity in the PbI₂.xMAI films(compared to the pure PbI₂ film) without forming the perovskiteCH₃NH₃APbI₃ phase.

In contrast to the slow conversion of planar PbI₂ film into CH₃NH₃PbI₃via the standard two-step sequential deposition, using the disclosedPbI₂.xMAI precursor films was found to convert to red-brown planarCH₃NH₃PbI₃ film relatively quickly when using the same sequentialdeposition processes. It took from about 1 minute to about 3 minutes forall PbI₂.xMAI (x: 0.1-0.3) films to convert completely into CH₃NH₃PbI₃without any PbI₂ residue as evidenced by the XRD measurements (FIG. 29).This contrasts significantly to the about 20 minutes to about 30 minutesrequired for the pure PbI₂ film to reach full conversion to CH₃NH₃PbI₃(FIG. 26b ) for the standard two-step sequential deposition process.FIG. 28c shows the UV-vis absorption spectra of the planar CH₃NH₃PbI₃films using various PbI₂.xMAI (x: 0.1-0.3) precursor films. Theabsorption spectrum for PbI₂ is also shown for comparison. Consistentwith their XRD results, all of these films exhibited the same absorptionshoulder near 750 nm, which is typical for CH₃NH₃PbI₃. The variation ofthe absorption intensity is presumably due to differences in CH₃NH₃PbI₃film thicknesses due to different precursors. A noticeable increase ofthe baseline was observed for the PbI₂.0.3MAI sample, corresponding to alarger light scattering effect as discussed in connection with FIG. 28a. Using the PbI₂.0.15MAI precursor film as an example, FIG. 26ddemonstrates the effect of MAI on the evolution of the absorptionspectra of the precursor films. The rapid increase of absorbance near750 nm indicated a quick conversion process from the PbI₂.0.15MAIprecursor film to the final perovskite CH₃NH₃PbI₃ film. Furthermore,there was no obvious increase of the baseline of the absorption spectraduring the entire conversion process, suggesting that the CH₃NH₃PbI₃film sequentially deposited from PbI₂.0.15MAI may have a similar surfaceroughness as the initial PbI₂.0.15MAI film.

FIGS. 30a-d show the typical SEM images of top views of the PbI₂.xMAI(x=0.1, 0.15, 0.2, and 0.3) precursor films. These films had verydifferent morphologies than the pure PbI₂ film and their morphologiesvaried with the amount of MAI used in the precursor. When x<0.2, theintroduction of MAI into PbI₂ led to the formation of a smootherPbI₂.xMAI film with less pinholes than the pure PbI₂ film. However, themorphologies of the PbI₂.0.3MAI films became even coarser than pure PbI₂films, which is consistent with the higher absorption baseline of thePbI₂.0.3MAI film as shown in FIG. 28a . FIGS. 30e-h show the typical SEMimages of the CH₃NH₃PbI₃ films prepared from PbI₂.xMAI films after thesecond contacting/conversion step. Note that the CH₃NH₃PbI₃ filmsfabricated from PbI₂.xMAI (x=0.1-0.2) were nearly pinhole free andshowed a relatively uniform, smooth surface, especially for the 0.15MAIsample, which was significantly different from the relatively roughCH₃NH₃PbI₃ films prepared from the pure PbI₂ film (See FIGS. 26a-d ).The grain sizes of the CH₃NH₃PbI₃ films also showed a dependence on theamount of MAI used in the precursor. When x=0.1, the CH₃NH₃PbI₃ filmconsisted mainly of relatively small CH₃NH₃PbI₃ nanocrystals with sizesless than 200 nm. When using 0.15-0.2 MAI, the CH₃NH₃PbI₃ films werecomposed of many relatively large (˜500 nm) crystals filled with ˜200 nmsmall crystals; such film morphology is comparable to that of thevapor-phase-grown perovskite films. This result is also consistent withthe absence of absorption baseline increase during the sequentialdeposition as shown in FIG. 28d . When starting with the relativelycoarse PbI₂.0.3MAI film, the CH₃NH₃PbI₃ film looked similar to thatprepared from the pure PbI₂ film. These results clearly demonstrate thatthe new PbI₂.xMAI-based modified three-step deposition method describedherein allows for the control of the morphology of the final CH₃NH₃PbI₃film.

In the standard two-step sequential solution deposition of CH₃NH₃PbI₃,the layer structured PbI₂ crystal was intercalated by MAI to form theCH₃NH₃PbI₃ as shown in FIG. 31a . This significant phase transformationnormally leads to approximately a factor of two volume expansion performula during the second conversion step. To help understand the effectof adding MAI (during the first step deposition) on the CH₃NH₃PbI₃ filmformation, the thickness (or volume) expansion ratios of convertingPbI₂.xMAI to CH₃NH₃PbI₃ films were examined by using a surface profiler.FIG. 31b shows the result of the expansion ratios for the films preparedwith different amounts (x) of MAI from 0 to 0.3. The ratio continuouslydecreased with increasing amount of MAI. Without wishing to be bound byany theory, it is believed that, for the PbI₂.xMAI films, the added MAIwas likely incorporated into the PbI₂ matrix and partially pre-expandedthe volume of PbI₂ matrix. The degree of this pre-expansion may dependon the amount of MAI used during the first step deposition. Thispre-expansion may decrease the final film expansion ratio withincreasing amounts of MAI. Assuming the pre-expansion ratio of reactantMAI is the same as the expansion ratio in the standard conversionprocess with pure PbI₂, the expansion ratio of the final CH₃NH₃PbI₃ filmthickness (d_(f)) to the thickness of the initial PbI₂.xMAI film (d_(i))can be calculated with the expression d_(f)/d_(i)=m/[1+(m−1)x], where mis the expansion ratio. A best fit of the data yields m=2.02±0.07; thecorresponding best-fit line is shown in FIG. 31b . The expansion isapproximately two, suggesting that the pre-expansion to form PbI₂.xMAIand the standard expansion to form CH₃NH₃PbI₃ have essentially the samevolume expansion ratio. Considering the absence of the crystallineCH₃NH₃PbI₃ phase in the PbI₂.xMAI films (FIG. 28b ), the additive MAIlikely forms an amorphous structure with the nearby PbI₂. The formationof this amorphous structure may strongly affect the crystallinity of theremaining PbI₂ phase. The XRD intensity of the PbI₂ peak is much reducedeven when the PbI₂ phase still constitutes the majority of the PbI₂.xMAIfilm (e.g., x=0.15; FIG. 28b ). Thus, the addition of small amounts ofMAI may result in several features that favor the rapid conversionreaction during the second step. These features may include theformation of amorphous PbI₂.MAI, the pre-expansion of the PbI₂ matrix,and reduced crystallinity of PbI₂. Moreover, using different amounts ofMAI may allow the control of the pre-expansion of the PbI₂.xMAIprecursor film, which may give rise to the advantageous morphology shownin FIGS. 30a-h and may also affect the final photovoltaic performance ofthe perovskite solar cells based on the planar CH₃NH₃PbI₃ films.

FIG. 32 summarizes photocurrent density-voltage (J-V) curves of planarCH₃NH₃PbI₃ solar cells prepared using different PbI₂.xMAI (x: 0-0.3)under simulated one-sun illumination. These PbI₂.xMAI films were dippedin the MAI solution for 2 min to form the CH₃NH₃PbI₃ layers, which wereadjusted to about 250 nm thick. In the absence of MAI (i.e., using thestandard PbI₂ precursor), the device shows a short-circuit photocurrentdensity (J_(sc)) of 8.847 mA/cm², open-circuit voltage (V_(oc)) of 0.969V, fill factor (FF) of 0.712, and overall conversion efficiency (η) of6.105%. The cell efficiency increases significantly when 0.1-0.2 MAIalong with PbI₂ is used during the first-step deposition, especiallywhen 0.15 MAI is used. A typical efficiency for the 0.15 MAI baseddevice is increased to 15.622% with the J_(sc) of 19.888 mA/cm², V_(oc)of 1.065 and FF of 0.738. The performance improvement is largelydetermined by the increased J_(sc) value associated with enhanced PbI₂conversion process and less perovskite damage resulting from minimizedinteraction with IPA solvent. The V_(oc) and FF values for the devicesbased on 0.1-0.2 MAI are also significantly larger than those of thecells based on the standard, pure PbI₂. However when 0.3 MAI is used,all device parameters are reduced with a typical J_(sc) of 13.867mA/cm², V_(oc) of 0.967 V and FF of 0.668, yielding an efficiency of8.977%. This is presumably due to the coarser morphology of CH₃NH₃PbI₃when 0.3 MAI is used, since there is no PbI₂ residue found in theperovskite film. The significant drop in cell performance of the 0.3 MAIbased device is consistent with its much reduced recombinationresistance (or faster recombination) compared to the 0.1-0.2 MAI-basedcells (FIG. 33). The details of photovoltaic parameters of all thesedevices including the statistical analysis (mean values and standarddeviations) for each type of devices are given in Table 6. Using 0.15MAI, the best cell efficiency recorded is 16.76% (FIG. 34a ). Themaximum power output of this device stabilizes at the efficiency ofabout 16.5% (FIG. 34b ), which is in good agreement with the valueobtained from the J-V measurement.

TABLE 6 Effect of PbI₂•xMAI precursor composition (x: 0-0.3) on thedevice parameters of typical planar perovskite CH₃NH₃PbI₃ solar cellsusing two-step sequential deposition. The mean values and standarddeviations of the device parameters from about 8 to 16 cells for eachtype of devices are given in parentheses. Precursor Type J_(sc) (mA/cm²)V_(oc) (V) FF η (%) PbI₂ (standard)  8.847 0.969 0.712  6.105  (9.067 ±0.790) (0.995 ± 0.036) (0.657 ± 0.069)  (5.914 ± 0.479) PbI₂ + 0.1MAI17.069 1.089 0.726 13.494 (18.009 ± 1.603) (1.097 ± 0.013) (0.714 ±0.039) (14.087 ± 1.208) PbI₂ + 0.15MAI 19.888 1.065 0.738 15.622 (20.388± 0.815) (1.085 ± 0.009) (0.714 ± 0.027) (15.798 ± 0.618) PbI₂ + 0.2MAI18.372 1.041 0.729 13.928 (18.526 ± 0.792) (1.056 ± 0.020) (0.721 ±0.063) (14.263 ± 1.253) PbI₂ + 0.3MAI 13.867 0.967 0.668  8.977 (14.013± 1.300) (0.949 ± 0.024) (0.642 ± 0.034)  (8.519 ± 0.712)

In summary, the present disclosure provides the use of a precursorcomposition comprising a mixture of PbI₂ and MAI (molar ratio 1:x, wherex varies from 0.1 to 0.3) to replace the pure PbI₂ used in the standardtwo-step sequential solution deposition of CH₃NH₃PbI₃. In comparison tothe standard two-step approach, the use of additive MAI during the firststep deposition leads to about 10-fold faster CH₃NH₃PbI₃ formationwithout any PbI₂ residue during the second MAI intercalation step andultimately results in a much improved device performance when 0.1-0.2MAI is used. The morphology of the CH₃NH₃PbI₃ film depends on therelative amount of MAI used in the PbI₂.xMAI precursor films during thefirst-step deposition, and generally demonstrates a smoother surfacemorphology when 0.1-0.2 MAI is used. Adding a small amount of MAI isfound to lead to the formation of amorphous PbI₂.MAI, the pre-expansionof the PbI₂ matrix, and reduced crystallinity of PbI₂, all of whichfavor the complete conversion within a short period of time and thusminimizes the negative impact of the IPA solvent on the integrity of theCH₃NH₃PbI₃ film during the second dipping/conversion step of thestandard two-step process. This new first-step precursor (PbI₂.xMAI) ispromising for facile preparation of high-quality planar CH₃NH₃PbI₃ filmswith controllable morphologies via the new two-step sequential solutiondeposition methods described herein, to fabricate high-performanceperovskite solar cells.

Experimental Setup and Device Preparation

CH₃NH₃I (MAI) was synthesized by reacting methylamine (33 wt % ethanolsolution) and hydroiodic acid (57 wt % in water, Aldrich) and purifiedas previously reported. Unless otherwise stated, the mixtures of 0.784 gPbI₂ (1.7 mmol) and 0, 0.027 g (0.17 mmol), 0.041 g (0.26 mmol), 0.054 g(0.34 mmol), or 0.081 g MAI (0.51 mmol) were dissolved in 2 mLdimethylformamide (DMF) at room temperature to form five differentprecursor solutions with mixed PbI₂ and MAI with a molar ratio of1:x—noted as PbI₂.xMAI (x=0, 0.1, 0.15, 0.2, 0.3, respectively).

A patterned fluorine-doped tin oxide (FTO) was first deposited with ablocking layer by spray pyrolysis at 450° C. using 0.2 M Ti(IV)bis(ethyl acetoacetate)-diisopropoxide 1-butanol solution, followed by450° C. annealing in air for one hour. The different PbI₂.xMAI filmswere prepared by spin coating different PbI₂.xMAI precursor solutions(pre-warmed at 65° C.) onto the substrate at 65° C. at 6,000 rpm for 15s. The spin coated PbI₂.xMAI films were dried in air for about 30-60 minbefore dipping in the 10 mg/mL MAI solution at 55° C. for differentduration. After quickly rinsing by anhydrous IPA twice, the CH₃NH₃PbI₃perovskite films were annealed on a hotplate at 150° C. for about 1 min,and then washed again by spin coating IPA at 4,000 rpm for 15 s,followed by an additional annealing at 150° C. for 1 min. Ahole-transport material (HTM) solution was spin-coated on theperovskite-covered TiO₂ electrodes at 4,000 rpm for 30 s. The HTMsolution consists of 0.065 M spiro-MeOTAD, 0.053 Mbis(trifluoromethane)sulfonimide lithium salt (Li-TFSi) and 0.20 M4-tert-butylpyridine (tBP) in chlorobenzene/acetonitrile (30:1, v/v)solution. Finally, a 150-nm-thick Ag layer was deposited on the HTMlayer by thermal evaporation. A typical cell area was about 0.12 cm² asdefined by a shadow mask.

The crystal structures of the perovskite films were characterized byX-ray diffraction (XRD, Shimadzu XRD-6100 diffractometer with Cu K_(α)radiation). The morphologies of the perovskite films were examined on aFEI Sirion 200 scanning electron microscope (SEM). The opticalabsorption spectra of CH₃NH₃PbI₃ perovskite films were measured by aUV/Vis spectrophotometer (Cary-60). The photocurrent-voltage (J-V)characteristic and continuous power output of perovskite CH₃NH₃PbI₃solar cells were measured respectively with a Keithley 2400 source meterand a potentiostat (Princeton Applied Research, VersaSTAT MC) under thesimulated AM 1.5G illumination (100 mW/cm²; Oriel Sol3A Class AAA SolarSimulator). Film thickness was measured by a surface profilometer(Dektak 8). Impedance spectroscopy (IS) was done using a PARSTAT 2273workstation with the frequency range of 0.1 Hz-100 kHz and themodulation amplitude of 10 mV. The IS spectra were analyzed using ZView2.9c software (Scribner Associates).

Example 5: Ammonia Sensing

It has been found that (CH₃NH₃)PbI₃ is not stable (with color bleaching)under polar solvents or in solutions containing high iodine/iodideconcentrations. Although the exact mechanism underlying this stabilityissue is unclear, the reported color bleaching for (CH₃NH₃)PbI₃ isirreversible. The (CH₃NH₃)PbI₃ film changes its color from brown toyellow when exposed to water, and cannot turn back from yellow to brownafter the removal of water. This disclosure shows that the NH₃ gasinduces a phase transformation of the perovskite (CH₃NH₃)PbI₃ film,leading to a rapid (<1 s) change of its color from brown to transparentacross the entire visible spectral range. This color change is reversedwithin seconds upon removing the NH₃ gas. These findings suggest thepotential use of perovskite halides (e.g., (CH₃NH₃)PbI₃) for NH₃ sensorapplications. It is believed that this special material property of(CH₃NH₃)PbI₃ or similar halide perovskites could inspire a broader rangeof applications and fundamental investigations.

The (CH₃NH₃)PbI₃ films were deposited onto mesoporous TiO₂nanocrystalline films on fluorine-doped tin oxide (FTO) by spin coatingusing a γ-butyrolactone solution comprising 35 wt % of stoichiometricPbI₂ and CH₃NH₃I, as described in previous reports. The effects ofammonia on the structural and optical properties of perovskite(CH₃NH₃)PbI₃ films were studied by X-ray diffraction (XRD, Rigaku D/Max2200 diffractometer with Cu K_(α) radiation) and ultraviolet-visible(UV-vis, Cary-6000i) absorption techniques. The transient photoresponseof (CH₃NH₃)PbI₃ associated with the introduction and removal of the NH₃source was probed with a 660-nm illumination using a silicon detectorconnected to a potentiostat (PARSTAT 2273).

FIGS. 35a-c show the sequence (left to right) of the color change of a(CH₃NH₃)PbI₃ film passing through the top opening of a bottle containinga 3% NH₃ solution. The perovskite film is deposited on a mesoporous TiO₂film on FTO. The sample is held at each position for about 1-2 seconds.Interestingly, we find that the brown (CH₃NH₃)PbI₃ film (FIG. 35a )immediately becomes nearly colorless when it is in contact with the NH₃gas (FIG. 35b ), and rapidly turns back to the brown color once the NH₃source is removed (FIG. 35c ).

FIGS. 36a-b includes (a) UV-vis absorption spectra of a (CH₃NH₃)PbI₃film in the absence and in the presence of NH₃ gas. (b) Photoresponse(transmitted light intensity) of a (CH₃NH₃)PbI₃ film at 660 nm asmeasured by a silicon detector in response to the introduction (on) andremoval (off) of NH₃ gas. FIG. 36a compares the UV-vis absorptionspectra of the perovskite (CH₃NH₃)PbI₃ film in the absence and in thepresence of NH₃. In the absence of NH₃, the (CH₃NH₃)PbI₃ film starts toabsorb around 750-800 nm and absorbs strongly below 600 nm, which agreeswith our previous study. In contrast, the absorption spectrum of the(CH₃NH₃)PbI₃ film across the entire visible region is essentiallyreduced to zero (i.e., the film becomes transparent in the visibleregion) when the perovskite film is exposed to the NH₃ gas.

The significant contrast of the optical absorption of the perovskite(CH₃NH₃)PbI₃ film in the presence and in the absence of NH₃ suggeststhat the (CH₃NH₃)PbI₃ film could potentially be used for optical NH₃sensing applications. Because the perovskite (CH₃NH₃)PbI₃ film has avery wide response over the entire visible spectral range (i.e., thedisappearance/substantial decrease of the entire absorption spectrumfrom 400 to 800 nm), the change of the absorption spectrum of the(CH₃NH₃)PbI₃ film at any single wavelength or wavelength range from 400to 800 nm can be used. FIG. 36b shows the photoresponse (transmittedlight intensity) at 660 nm as measured by a silicon detector. Thetransmitted light intensity increases immediately (<1 s) after theintroduction of NH₃ and also decreases quickly (1-2 s) after NH₃ isremoved. It is noteworthy that other perovskite halides (e.g.,(CH₃NH₃)PbI_(3-x)Cl_(x) and (CH₃NH₃)PbBr₃) exhibit similar opticalchanges in the presence of NH₃.

To understand the effect of NH₃ on the optical change of the perovskitefilm, we carried out XRD measurements on the perovskite samples in thepresence and in the absence of the NH₃ environment (FIG. 37). Forcomparison, the XRD patterns of the TiO₂/FTO substrate and PbI₂deposited on the TiO₂/FTO substrate are also shown in FIG. 37. Theperovskite (CH₃NH₃)PbI₃ film shows the standard XRD pattern, as wedescribed previously. Several characteristic peaks associated with theperovskite structure are denoted with asterisks. The XRD pattern of theperovskite film changes substantially after the film is exposed to theNH₃ gas. The characteristic XRD peaks (e.g., at 14.02°) for (CH₃NH₃)PbI₃either disappear or become indistinguishable from the background signal.Interestingly, an unknown peak at about 11.6° arises after the NH₃treatment. This characteristic peak cannot be assigned to either(CH₃NH₃)PbI₃ or PbI₂. It is also noteworthy that the XRD pattern of the(CH₃NH₃)PbI₃ film in the presence of NH₃ cannot be matched to otherpossible compounds such as NH₄PbI₃, Pb(NH₃)₄I₃ and NH₄PbI₃.xH₂O. Thus,further studies are required to identify the crystal structure of theperovskite film subjected to the NH₃ treatment. Based on the resultsabove, it is evident that the dramatic change of the optical property ofthe perovskite film results from its interaction with NH₃ at roomtemperature.

To our knowledge, previous studies provide virtually no information onthe phase transition of organometallic halide perovskites induced by NH₃at room temperature. It is well known that metal oxide perovskites(e.g., Sr₂RuO₄) are layered materials (due to their strong in-planechemical bonding and weak out-of-plane bonding) and thus can be easilyexfoliated by a solution process. The metal oxide perovskites areusually exfoliated through a liquid-phase transformative process viaion-exchange and intercalation reactions. PbI₂ also has a layeredstructure. It was shown previously that the yellow PbI₂ can turncolorless by reacting with nitrogen-containing molecules (e.g., ammoniaor pyridine) to form either an intercalation compound or a coordinationcomplex; these colorless products can largely reverse back to the yellowPbI₂ once the nitrogen source is removed. Inasmuch as (CH₃NH₃)PbI₃ showsessentially the same optical change as PbI₂ when NH₃ is introduced to orremoved from the perovskite film, we hypothesize that NH₃ eitherintercalates into or forms a new coordination complex with theperovskite network and that the crystal structure of (CH₃NH₃)PbI₃ canalso be largely restored once the NH₃ source is removed. It isnoteworthy that the (CH₃NH₃)PbI₃ film exposed to NH₃ for a long periodof time (e.g., tens of minutes) does not turn back to the brown colorwhen the NH₃ source is removed. The exact mechanism of NH₃-induced phasetransformation of (CH₃NH₃)PbI₃ is still uncertain. Further studies arerequired to understand how the crystal structure of (CH₃NH₃)PbI₃ evolvesin the presence of NH₃ and then how the crystal structure of NH₃-treated(CH₃NH₃)PbI₃ changes with the removal of the NH₃ environment.

In summary, we investigated the impact of a NH₃ gas environment on thestructural and optical properties of perovskite (CH₃NH₃)PbI₃. A mildtreatment of NH₃ to (CH₃NH₃)PbI₃ at room temperature strongly affectsits crystal structure and physical appearance. The color of the(CH₃NH₃)PbI₃ film changes immediately (<1 s) from brown to colorless(i.e., transparent from 400 to 800 nm) in the presence of NH₃. The colorof the NH₃-treated (CH₃NH₃)PbI₃ film turns back to brown within 1-2 safter the NH₃ environment is removed. XRD measurements show that thecrystal structure of perovskite (CH₃NH₃)PbI₃ changes significantly whenthe perovskite film is exposed to NH₃. This structure change couldresult from the formation of either an intercalation compound or acoordination complex from (CH₃NH₃)PbI₃ and NH₃. The results of thisstudy imply that organometallic halide perovskites (e.g., (CH₃NH₃)PbI₃)could potentially be used as ammonia sensors with both a fast responsetime and a wide range of spectral response. Thus, understanding howhalide perovskites interact with their surrounding environment will helpresearchers fully use the unique properties of this new absorber systemfor solar cell applications, but may also lead to potential applicationsof these materials in different fields.

Example 6: Molybdenum Oxide Top Contact

At present, perovskite solar cells are often made with two types ofcommon device architectures, i.e., either mesostructured or planarthin-film solar cells. The first type is similar to the conventionalsolid-state dye-sensitized solar cells (DSSCs). A key component of thisdevice configuration is a mesoporous metal-oxide (e.g., TiO₂) layer thatis first coated with nanostructured perovskite absorbers and theninfiltrated with a hole transport material (HTM; e.g., spiro-MeOTAD). Inaddition to being the light absorber, perovskite can also be used as ahole conductor. In this situation, the standard HTM layer can beavoided. For the planar perovskite solar cells, a thin layer (a fewhundred nanometers) of perovskite absorber is sandwiched between theelectron- and hole-contact layers (e.g., TiO₂ and spiro-MeOTAD,respectively). For both the mesostructured and planar perovskite solarcells, a layer (about 80-150 nm thick) of Au or Ag is often used as thetop metal contact for carrier extraction. Replacing Au or Ag with alow-cost material/structure that can also effectively extractphotogenerated holes would help reduce the overall cost for producingperovskite solar cells.

This disclosure demonstrates that a thin layer (10 nm) of molybdenumoxide (MoO_(x)) coupled with Al can be used as an effective top-contactstructure for extracting photogenerated holes from perovskite CH₃NH₃PbI₃solar cells. In recent years, transition metal oxides (TMO; e.g.,MoO_(x)) have been used as either an interlayer or buffer layer for avariety of optoelectronic devices to improve either hole injection(e.g., in organic light-emitting diodes) or hole extraction (e.g., inorganic photovoltaics, CdTe, Si, and quantum-dot solar cells). We findthat the device performance of perovskite solar cells using a MoO_(x)/Altop contact is comparable to that of cells using the standard Ag topcontact. Analysis of impedance spectroscopy measurements shows thatusing 10-nm-thick MoO_(x) and Al does not affect charge-recombinationproperties. However, a thicker (20-nm) MoO_(x) layer leads to decreasedcell performance resulting primarily from a reduced fill factor.

The precursor of CH₃NH₃I was synthesized and purified as previouslyreported. The pre-patterned fluorine-doped SnO₂-coated glass substrate(FTO; TEC15, Hartford, USA) was coated with a compact TiO₂ layer byspray pyrolysis using 0.2 M Ti(IV) bis(ethylacetoacetate)-diisopropoxide in 1-butanol at 450° C. A mixture of PbCl₂and CH₃NH₃I (1:3 molar ratio) was dissolved in dimethylformamide (DMF)to form the precursor solution, which was spin-coated onto the TiO₂/FTOsubstrate at 2500 rpm for 10 s, followed by annealing at 100° C. for 45min in air. A spiro-MeOTAD-based HTM solution with a previously reportedrecipe was spin-coated on the perovskite-covered TiO₂ electrodes at 4000rpm for 30 s. A thin layer (0-20 nm) of MoO_(x) was deposited by thermalevaporation. Finally, a 150-nm-thick metal (Ag or Al) layer wasdeposited by thermal evaporation. The active area of each device wasabout 0.2-0.3 cm². The X-ray diffraction (XRD) patterns of theperovskite films were conducted using Rigaku D/Max 2200 diffractometerwith Cu K_(α) radiation. The ultraviolet/visible (UV/vis) spectra werecharacterized by an ultraviolet/visible-near infrared (UV/vis-NIR)spectrophotometer (Cary-6000i). The morphology of perovskite film wasexamined by a field-emission scanning electron microscopy (FE-SEM, JEOLJSM-7000F). The photocurrent density-voltage (J-V) characteristic ofperovskite solar cells were studied using a class AAA solar simulator(Oriel Sol3A). Impedance spectroscopy (IS) was done with a PARSTAT 2273workstation with the frequency range of 0.1 Hz-100 kHz and modulationamplitude of 10 mV.

FIG. 38 displays the XRD patterns of a perovskite CH₃NH₃PbI₃ filmdeposited on a compact TiO₂ layer on the FTO-coated glass substrate. Weobserve characteristic perovskite diffraction peaks (denoted by stars)at about 14.1, 28.44, 31.88, and 43.24°, respectively, corresponding tothe diffractions from (110), (220), (310), and (314) crystal planes ofthe CH₃NH₃PbI₃ perovskite structure. The positions of these diffractionpeaks are consistent with previous reports. The other XRD peaks areattributable to the TiO₂/FTO substrate. No secondary phases (e.g., PbI₂)are found. The typical UV-vis absorption spectrum for the sameperovskite CH₃NH₃PbI₃ film is shown in the inset of FIG. 38. Consistentwith our previous reports, the CH₃NH₃PbI₃ film strongly absorbs theillumination below 600 nm. The absorbance of CH₃NH₃PbI₃ decreasesslightly with increasing wavelength from about 600 to 750 nm and thendrops rapidly when the wavelength approaches the bandgap near 800 nm.

FIG. 39a shows the typical cross-sectional SEM image of the planarperovskite CH₃NH₃PbI₃ solar cell with the following layered structure:FTO/compact TiO₂/perovskite absorber/spiro-MeOTAD/MoO_(x)/metal. In thisstudy, the metal layer is either Ag or Al. Although it is not evident inthe SEM image, a 10-nm MoO_(x) interlayer is deposited by thermalevaporation between the spiro-MeOTAD and metal layers. The averagethickness of the CH₃NH₃PbI₃ layer is about 300 nm, and the averagethickness of the HTM layer (spiro-MeOTAD) is about 280 nm. In FIG. 39b ,we test the effect of MoO_(x) interlayer thickness on the J-Vcharacteristics of perovskite CH₃NH₃PbI₃ solar cells using the Ag topcontact. In the absence of MoO_(x), the device displays a short-circuitphotocurrent density (J_(sc)) of 19.94 mA/cm², open-circuit voltage(V_(oc)) of 1.011 V, and fill factor (FF) of 0.613 to yield anefficiency (η) of 12.36%. When 5-nm and 10-nm MoO_(x) layers are used,the respective efficiencies are 11.49% and 12.04%. The mean values andstandard deviations of the PV parameters from 8-20 cells for thesedevice architectures are given in Table 7. It is evident that the 5-nmand 10-nm MoO_(x) cells have essentially the same performance level asthe cell without the MoO_(x) layer, within experimental error. However,when a 20-nm-thick MoO_(x) layer is used, the cell efficiency issignificantly reduced from about 12% to 9.33%. The decrease of cellefficiency is primarily caused by the much reduced FF, which is inagreement with a recent study on the effect of MoO_(x) layer thicknesson the device characteristics of Si solar cells. The details of thephotovoltaic parameters (J_(sc), V_(oc), FF, and η) for all these cellsare given in Table 7.

TABLE 7 Effect of the top layer (hole-contact) structure on theshort-circuit photocurrent density J_(sc), open-circuit voltage V_(oc),fill factor FF, and conversion efficiency η of champion planarperovskite CH₃NH₃PbI₃ solar cells. The mean values and standarddeviations of the PV parameters from 8-20 cells for each type of devicesare given in parentheses. Cell Type J_(sc) (mA/cm²) V_(oc) (V) FF η (%)HTM/Ag 19.94 1.011 0.613 12.36 (19.45 ± 0.90) (0.967 ± 0.040) (0.538 ±0.043) (10.13 ± 0.99)  HTM/5-nm MoO_(x)/Ag 19.60 0.985 0.595 11.49(19.43 ± 0.60) (0.952 ± 0.026) (0.544 ± 0.027) (10.08 ± 0.81)  HTM/10-nmMoO_(x)/Ag 20.14 0.993 0.602 12.04 (20.37 ± 0.95) (0.962 ± 0.021) (0.543± 0.036) (10.63 ± 0.70)  HTM/20-nm MoO_(x)/Ag 19.56 1.008 0.473  9.33(19.22 ± 0.82) (0.964 ± 0.024) (0.439 ± 0.025) (8.15 ± 0.64) HTM/10-nmMoO_(x)/Al 19.55 0.990 0.590 11.42 (19.31 ± 0.85) (0.972 ± 0.023) (0.542± 0.030) (9.85 ± 0.91) HTM/Al  0.92 0.182 0.165  0.027  (0.67 ± 0.26)(0.119 ± 0.104) (0.203 ± 0.060) (0.015 ± 0.011) 10-nm MoO_(x)/Ag  0.840.045 0.261  0.01  (0.66 ± 0.13) (0.034 ± 0.016) (0.254 ± 0.025) (0.006± 0.003)

Having established that MoO_(x) works as an effective interlayer forhole extraction in our regular device architecture, we compare in FIG.40a the J-V characteristics of perovskite CH₃NH₃PbI₃ solar cells usingdifferent top-contact structures, including: (1) HTM/Ag, (2)HTM/MoO_(x)/Ag, (3) HTM/MoO_(x)/Al, (4) HTM/A1, and (5) MoO_(x)/Ag (noHTM). For top-contact structures (2), (3), and (5), we used a fixed10-nm-thick MoO_(x) layer. The composition of the spiro-MeOTAD-based HTMlayer is given in detail in the experimental section. Using theHTM/MoO_(x)/Al contact structure, the cell efficiency is 11.42% with aJ_(sc) of 19.55 mA/cm², V_(oc) of 0.990 V, and FF of 0.590. All of theseparameters are comparable to those of the cells using HTM/Ag andHTM/MoO_(x)/Ag contact structures (Table 7). Consistent with the J-Vresults, the incident photon-to-current efficiency (IPCE) spectra forcells based on contact structures (1)-(3) are essentially identical(FIG. 40b ). These results suggest that the junction between MoO_(x) andthe metal contact is not critical to hole extraction in perovskite solarcells. This observation agrees with other studies where MoO_(x) is usedas an interlayer in organic solar cells and quantum-dot solar cells²⁸ tofacilitate extraction of photogenerated holes. It is worth noting thatthese studies demonstrated improved cell performance using the MoO_(x)interlayer than the control sample without the MoO_(x) layer. Incontrast, our study shows that cells using HTM/MoO_(x)/Ag orHTM/MoO_(x)/Al have comparable performance to the control cells usingHTM/Ag. This is presumably caused by the use of HTM in this study,whereas in other studies the MoO_(x) layer is deposited directly on theabsorber layer. Interestingly, we find that when Al is depositeddirectly on top of the HTM layer (i.e., no MoO_(x) is used), theperovskite solar cell only shows a conversion efficiency of 0.027% witha J_(sc) of 0.92 mA/cm², V_(oc) of 0.182 V, and FF of 0.165. This couldbe attributed to the formation of a back diode at the HTM/Al interface,which creates an energy barrier limiting hole extraction. It was shownpreviously for PbS quantum dot solar cells that the back-diode effectcan be significantly reduced when using higher work-function metals(e.g., Au or Ag) or can be completely removed by using a MoO_(x)interlayer. It is worth noting that using MoO_(x)/Ag directly on theperovskite absorber layer (i.e., no HTM layer) leads to a 0.01% device(Table 7). This is presumably caused by shorting associated withpinholes on the absorber layer. The use of the HTM layer appears to beeffective in suppressing shorting in perovskite cells.

Impedance spectroscopy is used to study the impact of top-contactstructure on the recombination resistance (R_(rec)) for the perovskitesolar cells. FIG. 41a shows the typical Nyquist plots of the compleximpedance Z (i.e., the imaginary component Im [Z] versus the realcomponent Re [Z] of the impedance) for a perovskite cell with threedifferent bias voltages. The impedance spectra are dominated by a largesemicircle at low frequencies. This feature has been attributed tocharge recombination either at the interface between perovskite andcontact layers or within the bulk perovskite layer. A larger semicirclecorresponds to a larger recombination resistance R_(rec), which isinversely proportional to the recombination rate. The model used forimpedance analysis has been previously discussed in detail by others.From these Nyquist plots, the recombination resistance of the perovskitecells can be determined. FIG. 41(b) shows the R_(rec) values as afunction of voltage for perovskite solar cells using the top-contactstructures (1)-(3). The R_(rec) for all these cell structures dependsstrongly on the bias voltage, following an approximately exponentialdecrease with voltage. Similar voltage dependence of recombinationresistance for perovskite solar cells has been reported previously.Interestingly, there is not much difference in recombination resistanceamong the three samples despite the significant difference in theirtop-contact structures, which suggests that the MoO_(x)/metal interfacedoes not affect charge transfer at the back contact. This is inagreement with the J-V measurements in FIG. 40a . Thus, the J-V andimpedance results suggest that MoO_(x)/Al can be used as an effectivehole-extraction contact to replace more expensive metal contacts (e.g.,Ag or Au) for perovskite solar cells.

In summary, we demonstrate the effectiveness of using a combination of athin layer of molybdenum oxide and aluminum as the top-contact structurefor extracting photogenerated holes from perovskite solar cells. MoO_(x)is deposited by thermal evaporation. The device performance ofperovskite solar cells using a MoO_(x)/Al top contact is comparable tothat of cells using the standard Ag top contact. Impedance measurementssuggest that the extraction of photogenerated holes is not affected bythe MoO_(x)/metal interface when proper MoO_(x) thickness (e.g., 10 nm)is used. Using a thicker (20-nm) MoO_(x) layer leads to decreased cellperformance resulting primarily from a reduced fill factor.

FIG. 42 shows the pictures of the perovskite solar cells after 15 hourcontinuous light soaking under simulated one sun illumination. Thedevices using MoO_(x) showed essentially no color change of theilluminated areas. In contrast, the device without MoO_(x) layer showedsignificant color change from brown to yellow, suggesting thedegradation of the perovskite layer under illumination.

FIG. 43 shows the dependence of solar cell efficiencies as a function ofcontinuous light soaking under simulated one sun illumination for 15hours. The efficiencies are normalized for easy comparison. The devicesusing MoO_(x)/Al layer showed the best devices stability among the cellstested.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub combinations thereof. It is thereforeintended that the following appended claims and claims hereafterintroduced are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truespirit and scope.

Finally, it should be noted that there are alternative ways ofimplementing the embodiments disclosed herein. Accordingly, the presentembodiments are to be considered as illustrative and not restrictive,and the claims are not to be limited to the details given herein, butmay be modified within the scope and equivalents thereof.

From the foregoing description, various modifications and changes in thecompositions and methods of the present disclosure will occur to thoseskilled in the art. All such modifications coming within the scope ofthe appended claims are intended to be included therein.

What is claimed is:
 1. A method comprising: applying a solution onto asubstrate, the solution comprising a first alkyl ammonium halide havinga first halogen, a second alkyl ammonium halide having a second halogen,and a metal halide in a solvent; and thermally treating the solution,wherein: the first halogen is different than the second halogen, thethermally treating results in an alkyl ammonium metal halide perovskitefilm on the substrate, and the alkyl ammonium metal halide perovskitefilm includes only one of the first halogen or the second halogen. 2.The method of claim 1, wherein during the thermally treating, anintermediate alkyl ammonium metal halide perovskite film formscomprising the first halogen and the second halogen.
 3. The method ofclaim 2, wherein the intermediate alkyl ammonium metal halide perovskitefilm comprises iodide.
 4. The method of claim 3, wherein theintermediate alkyl ammonium metal halide perovskite film compriseschloride and iodide.
 5. The method of claim 4, wherein the intermediatealkyl ammonium metal halide perovskite film comprises at least one of amethyl ammonium metal iodide and a methyl ammonium metal chloride, or amethyl ammonium metal dihalide comprising iodide and chloride.
 6. Themethod of claim 5, wherein the alkyl ammonium metal halide perovskitefilm comprises a methyl ammonium metal iodide.
 7. The method of claim 1,wherein an alkyl group for at least one of the first alkyl ammoniumhalide or the second alkyl ammonium halide comprises at least one of amethyl group, an ethyl group, a propyl group, or a butyl group.
 8. Themethod of claim 1, wherein both the first halogen and the second halogencomprise at least one of fluorine, chlorine, bromine, or iodine.
 9. Themethod of claim 8, wherein the first halogen comprises chloride and thesecond halogen comprises at least one of bromine, or iodine.
 10. Themethod of claim 1, wherein the metal halide comprises at least one oflead, tin, germanium, or any other metal in a 2+ valence state.
 11. Themethod of claim 1, wherein the solvent comprises at least one polarsolvent.
 12. The method of claim 11, wherein the solvent comprises atleast one of dimethysulfoxide, dimethylformamide, γ-butyrolactone, orN-methyl-2-pyrrolidone.
 13. The method of claim 1, wherein the metalhalide and the second alkyl ammonium halide are present in the solutionat a molar ratio of about one to one (1:1).
 14. The method of claim 13,wherein the second alkyl ammonium halide and the first alkyl ammoniumhalide are present in the solution at a molar ratio between about 1:0.1and about 1:4.
 15. The method of claim 14, wherein the first alkylammonium halide is an alkyl ammonium chloride and the second alkylammonium halide is an alkyl ammonium iodide.
 16. The method of claim 1,wherein at least one of the first alkyl ammonium halide, the secondalkyl ammonium halide, or the metal halide are present in the solutionat a concentration between about 0.1 M and about 3.0 M.
 17. The methodof claim 1, wherein the thermally treating comprises heating thesolution at a temperature between about 20° C. and about 250° C.
 18. Themethod of claim 1, wherein the thermally treating comprises heating thesolution for a period of time between about 30 seconds and about 6hours.